Take a verb, put can in front, and the verb loses its endings. She walks, but she can walk. He went, but he could go. The helping verb has taken the work, and the main verb is left bare.
There are nine words that do this, and they are called modals: can, could, may, might, must, shall, should, will and would. They add a meaning to the verb after them — whether it can, may or must happen — and they have no forms of their own.
Why nothing else is needed
A clause shows its tense once. In she could go, the tense is in could. Add an ending to go and the tense is shown twice. That is the mistake in she can goes: the speaker has put the third person ending on a verb that follows a word which has already taken it.
Counting it
The lab finds every modal in Chapter XXVI and reads what follows. There are 76. A bare verb follows 65 times, which is 85.5%. Of those 65, 20 have not or an adverb between the modal and the verb: you could not do better, we shall often meet. The bare form holds through the gap.
The test of the rule is a row that would break it: a modal followed by a verb with -s, -ed or -ing. The lab has a box for that, and in the chapter it holds 0.
The eleven others
Four rows have a pronoun right after the modal. Two are questions, will you come and how can I promise: the bare verb comes after the subject, so the rule holds, but the scan reads one word and files it apart. Two are not. In he should not. I see the modal ends its sentence and the scan read on past the full stop; in will, I am sure, be a remark stands between the modal and its verb. Seven rows are filed as something else. Look at them: two have a bare verb the word list does not know, one has a bare verb behind but, one has the verb left out, and three put no longer or at present in the way. None of them has an ending on the verb.
What a whole novel says
Counted once over the whole novel, which no page can carry, and quoted here, there are 2,738 modals. A bare verb comes directly after 58.5% of them. Not comes between in 14.9%, and an adverb in 9.5%. A pronoun comes next, a question, in 5.2%. A verb with an ending follows 3 of the 2,738, which is 0.1%, and the three are worth naming: as soon as he could, provided he chose; as well as she could, said that; as soon as we can, said Jane. In each the modal ends its clause, and the verb after the comma belongs to the next one. The rule has no true exception in 2,738 rows, and that is a rule worth stating simply.
The two modern documents give 16 of 17 rows, 94.1%, and again no verb with an ending. The one row that is not a hit is the month May in changes in May, which the scan reads as the word may.