Forum: ell.stackexchange.com
Tags: <modal-verbs><infinitives><conjugations>
Title: "It can contain" or "It can contains"?
Question: What is the correct form?    1) It can contain  2) It can contain s

Answers:
sents: text: As a native speaker I would say 1)
text: it can contain To use contains, the word "can" must be dropped, e.g. 2b)
text: it contains Only the modal verb (in this case, can) has to be in third person singular, the second part of the verb must be in bare infinitive form (because it is used after a modal).
answer_details: author: ell.stackexchange.com/users/24688/Ron Jensen - We are all Monica
score: 13
sents: text: The modal verbs in English have a very strict grammar.
text: The main members of this category are: Can, could, shall, should, will, would, may , might, must These are the most important things to know about modal verbs:
text: They have no other forms apart from the forms you see above.
text: The following words, for example, are impossible in English: cans , canning , canned , to can Modal verbs never occur with other modal verbs in the same clause.
text: The following sentence is ungrammatical because it uses two modals together: *
text: When I finish my studies, I will can speak
text: English perfectly Modal verbs are always the first verb in the clause.
text: The following is ungrammatical: *
text: She have been could studying English.
text: Point number (4) is the most important one for the Original Poster's question: Modal verbs are always followed by a verb in the plain form.
text: This is the form you see in a dictionary.
text: The following sentences are ungrammatical.
text: They are impossible in English: *
text: She can dances.
text: *
text: She can dancing.
text: *
text: She can to dance.
text: *
text: She can danced.
text: For this reason the Original Poster's example must be like this: She can dance.
text: For the nine central members of this class these rules are always true.
text: There are never any exceptions.
text: Ever.
text: If we break any of these rules, our sentence is guaranteed to be ungrammatical.
text: It's good to have some rules in English that don't have exceptions!
answer_details: author: ell.stackexchange.com/users/8084/Araucaria - Not here any more.
score: 10
sents: text: After modal verbs follows a bare infinitive.
answer_details: author: ell.stackexchange.com/users/4159/rogermue
score: 7
sents: text: First one is the correct form.
text: http://www.englishpage.com/modals/can.html
text: http://www.perfect-english-grammar.com/modal-verbs.html
text: If you want to use contains, drop can...
answer_details: author: ell.stackexchange.com/users/23935/RajSharma
score: 2
sents: text: Ron Jensen is correct, but fails to point out that the two phrases have different meanings: 1) it can contains -- makes no sense / is incorrect 2) it contains -- means there is some thing inside "it" 3)
text: it can contain -- means that "it" is capable of holding some thing Example: If you are speaking of a 100ml drinking glass: If the glass is half full, you might say: "It contains 50ml of water." or
text: If you are speaking of its ability to hold some amount, you might say: "It can contain 75ml."
text: If you want to tell the absolute maximum you need some modifier, you might say: "It can contain up to 100ml."
answer_details: author: ell.stackexchange.com/users/26175/NJRandy
score: 2
sents: text: 1) It can contain is the correct form.
text: I also agree with Ron Jensen's for "It contains", but here the phrase suggests it is definite as opposed to the original phrase "It can contain".
answer_details: author: ell.stackexchange.com/users/26162/theDarkPriest
score: 1
sents: text: After a model verb, the verb must be in bare-infinitive form.
text: So, "It can contain" is correct.
text: "It can contains" must be modified to "It can contain", or "It contains".
answer_details: author: ell.stackexchange.com/users/26174/user26174
score: 1

Summary of the answers:
"It can contain" is the correct form.   
It can contain is the correct form.