What is the correct form?
1) It can contain
2) It can contains
As a native speaker I would say
1) it can contain
To use contains, the word "can" must be dropped, e.g.
2b) 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).
The modal verbs in English have a very strict grammar. The main members of this category are:
These are the most important things to know about modal verbs:
They have no other forms apart from the forms you see above. The following words, for example, are impossible in English:
Modal verbs never occur with other modal verbs in the same clause. The following sentence is ungrammatical because it uses two modals together:
Modal verbs are always the first verb in the clause. The following is ungrammatical:
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. This is the form you see in a dictionary. The following sentences are ungrammatical. They are impossible in English:
For this reason the Original Poster's example must be like this:
For the nine central members of this class these rules are always true. There are never any exceptions. Ever. If we break any of these rules, our sentence is guaranteed to be ungrammatical. It's good to have some rules in English that don't have exceptions!
After modal verbs follows a bare infinitive.
First one is the correct form.
If you want to use contains, drop can...
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) it can contain -- means that "it" is capable of holding some thing
If you are speaking of a 100ml drinking glass:
If the glass is half full, you might say:
"It contains 50ml of water."
If you are speaking of its ability to hold some amount, you might say:
"It can contain 75ml."
If you want to tell the absolute maximum you need some modifier, you might say:
"It can contain up to 100ml."
After a model verb, the verb must be in bare-infinitive form.
1) It can contain is the correct form.
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".