Some background.
Sentences are based upon physics: Why? Because even if a story is about sentient pickup trucks floating in a different dimension, it is written in language which evolved expressing human experiences utilizing the neural pathways of our species. All complete sentences need to inform the reader of what’s happening in terms of the three major elements of physics: matter, time, and energy. Often, that’s dead simple to figure out, but it can get a tad tricky.
The Representation of Time, Matter, and Energy in English Sentences.
A. Time has two providers in English, 1) the verb and 2) the auxiliary or modal.
Your third grade teacher may have called auxiliaries and modals ‘helping verbs,’ though they really aren’t verbs at all. Instead, they work with verbs to more clearly position things in time.
Verbs in English are a bit unusual in two ways. First, English provides very thin time information by way of inflections, which usually ride at the end of the word. In essence, there are only three beyond the infinitive to form (to see, to play, to examine). Many other languages are richer in time designations riding directly on the verb.
- past: -ed
- present: -Ø, -s
I see, you, we, they see, etc.; but he, she, or it sees - ongoing present: -ing
- This does not play out directly. An -ing ending on a verb all by itself creates a gerund (a verb functioning as a noun) as in, “Jenny enjoys [the verb] cooking gourmet vegan meals [the direct object] for her friends [the indirect object].
- For -ing forms to function as verbs, they must be preceded by auxiliaries: I am seeing, he is seeing, they were seeing, etc.
Auxiliaries and modals. In English, most of the details of time is provided by way of auxiliaries.
- Auxiliaries have several tenses, or time orientations.
- Present continuous action (Present Progressive)
= is + [verb + -ing] - Col. Mustard is killing the butler in the pantry with a bread knife. (It took awhile.)
- I am defending Col. Mustard at trial.
- Past continuous action (Past Progressive)
= was + [verb + ing] - Yesterday, Col. Mustard was stalking the butler.
- He was polishing the bread knives after the time of the murder, perhaps to wipe off any fingerprints.
- The guests were walking the halls all night, whispering to one another, getting their stories straight.
- The recent past (present perfect) = a form of have + [verb + -ed].
- Col. Mustard has killed the butler.
- The alternate has joined the jury for that trial.
- Col. Mustard has hired a good attorney.
- The distant past (past perfect)
= had + [verb + ed] - Jill had promised to carry the water on her own.
- Sometimes, the verbs are irregular, as in this example: The prosecuting attorney had given a strong summation.
- Sometimes the spelling varies, as when the -y is changed to -i- before the -ed is added deny, denied. And, sometimes an adverb sneaks into the mix: Jack had strongly denied the crime.
- Unlike auxiliaries, modals never change form to agree with the subject. They include can, could, may, might, must, ought, shall, should, will, and would. Modals never change form. When they occur with a verb, the third person -s in the present tense and the -ed in the past tense of the verb are both dropped.
- He plays the lute. -> add modal = He can play the lute.
- Not: *He cans play the lute.
or *He can plays the lute.
[Note: The asterisk denotes either an ‘illegal’ or a historical (no longer in use) form.
Unlike other verbs, modals
- never change their forms because of time, number, etc., and
- are never followed by to with the exception of ought to.
Like auxiliaries, modals need additional verbs to express just what needs to happen.
- I can eat corn with these dentures!
- I must submit my taxes before the deadline.
- Col. Mustard may kill the butler.
Modals do not need additional auxiliaries in negatives or questions. Just move the modal forward in questions.
Examples:
- You should not eat that.
Should you do that? - The puppy could eat that for me.
Could the puppy share that with me?
[No, chocolate will poison your puppy!] - I could have eaten.
NOT *I could of eaten. - Some others:
I must have overslept.
I ought to have tipped more.
I have got to do my homework now.
I might kill the butler, or I might go on a picnic with Mrs. Peacock. It just depends on what I’m in the mood for tomorrow.
B. Mood. In addition to tenses, which tell time, most Indo-European languages have verb moods. In English, these are the big five moods:
- Indicative (factual and real). Ms. Scarlett sits in the witness chair.
- Imperative (command). “Sit!” the judge told the defense attorney.
- Interrogative (questioning). “Would you sit there?” the attorney asked his client.
- Conditional (if, then, else). My client might be more willing to sit if a shorter chair were substituted so her feet could touch the ground.
- Subjunctive (hypothetical). If I were you, I’d sit down and shut up, missy!
Some others:
- Subjunctive: Marie Antoinette said, “Let them eat cake.”
Indicative: The peasants ate only dandelion greens and grass clippings.
Imperative: Eat!
Conditional: If you were to eat only cake, you would probably get scurvy.
Conditional: I might be willing to eat those greens if there’s cake for dessert. - Subjuncive: Yeh, and if I were a bird, I’d fly! (sarcasm)
Indicative: In a former life, I was a bird. - Subjunctive: If I were to teach online, I would need to build a website first.
Indicative: If he teaches online, he first needs to build a website.
- Subjunctive: After a big sneeze… God bless you! (This doesn’t speak to what god’s doing at the moment. This is a hypothetical or hopeful wish.
Indicative: The priest said, “bless you my child.” - Subjunctive: How might I help you? [I’m not…at least not yet… Just askin’…]
Indicative: These instructions will help you put that bike together. [That’s more likely than purely hypothetical.] - Subjunctive: I suggest that Col. Mustard hide the murder weapon.
Indicative: Col. Mustard hides the murder weapon. - Subjunctive: Col. Mustard thought he’d feel better if he were to kill the butler.
Indicative: When Col. Mustard kills the butler, his problems are just getting started.
C. Matter, real or imagined is displayed through its noun(s) and extends to include those things that describe them. Sometimes that’s one word, and sometimes it’s an extended phrase. Often, subject verb agreement depends upon keeping track of just how many of the critters you’re talking about.
ONE SINGLE THING:
I climb[Ø] the hill.
Jill is climbing the hill.
Jill climbs the hill.
Ignore the phrase which comes between subject and verb:
- One [of the animals] is climbing the hill.
Each one climbs the hill. - But: They [each think it through and then] climb[Ø] the hill.
They all climb[Ø].
Some mass and collective nouns are treated as singular:
- Collective: This includes team, committee, family, crew, regiment, pride (of lions), pack (of wolves), etc. Note: whether something is considered an aggregate plural or singular varies by country. The Brits, for example, will say, “the team are fighting amongst themselves,” while the Yanks prefer is (and among).
- Mass aggregate: The rice, oatmeal, etc., is cooked, but the potatoes are cooked. The snow is heavy today, but the snowflakes are piling up. [See multipart units like scissors and soap suds under plurals.]
Two or more singulars are treated as one when connected by or or nor:
- The pony or the goat climbs the hill.
- Neither the pony nor the goat climbs the hill anymore.
But: Both are too old. None of the ponies or goats can climb[Ø] the hill anymore.
The following words are considered singulars for the purpose of subject/verb agreement: anybody, anyone, each, each one, either, everyone, everybody, neither, no one, nobody, somebody, someone.
- Each of these animals is old and decrepit.
- Nobody likes a decrepit goat (except his friend, the pony).
- “Everybody loves a lover,” said the pony, nuzzling the goat.
The noun is the noun is the noun, so ignore asides such as together with, in addition to, accompanied by, etc. Examples:
- The team, accompanied by the band and cheerleaders, is being bused to the event. (If you’re in the U.S. Brits, of course, would say are.)
- The books, including that over-sized one nobody can lift, are being stored in a freezer to dry them out after the flood.
- President Anderson, accompanied by his wife, is attending the concert.
Money, money, money, money!
- When talking about an amount of money, treat it as singular. Twenty dollars is not a lot of money these days.
- Referring to the bills (currency) themselves, use the plural: They would have taken Bitcoin, but he paid for the rare blue diamond in dollars, which are always welcome.
MORE THAN ONE THING:
- Verbs agree with plural nouns:
- Jack and Jill are climbing the hill.
They are climbing.
- Jack and Jill climb[Ø] the hill.
They climb[Ø] quickly.
- But: Ignore the phrase which comes between subject and verb:
The goats [who can still climb the hill at this altitude] are panting desperately. 
Things with more than one part are treated as plural: clothes, scissors, pants, glasses, shorts, jeans, leggings, emissions, earnings, binoculars, snippits, forceps, jitters, suds (we can have one bubble, but not one ‘sud’), shenanigans, remains, annals, riches, underwear, panties, tweezers, trousers, sheers.- Which comes first matters: When there are compounds with a singular and a plural, make the verb match the one closer to it.
- The gym teacher and her students climb[Ø] the hill every day.
- Either one of the students or the teacher[Ø] climbs the hill to get water every morning before school.