The third person -s.
Hello everyone.

I usually add '3rd person -s' like these:
He/She/It sees.
He/She/It hears.
He/She/It walks.

But, why shouldn't we add the '3rd person -s' to the verb 'Do' here?:
- He can do everything.
Why not 'He can does everything'?

http://www.italki.com/entry/562517

Compare:
- He does his homework.
- He can do his homework.

I don't know the reason.
Both of them are 'Main verb', not 'Auxiliary verb'. I mean the verb 'Do'.

Question

The third person -s.
Hello everyone.

I usually add '3rd person -s' like these:
He/She/It sees.
He/She/It hears.
He/She/It walks.

But, why shouldn't we add the '3rd person -s' to the verb 'Do' here?:
- He can do everything.
Why not 'He can does everything'?

http://www.italki.com/entry/562517

Compare:
- He does his homework.
- He can do his homework.

I don't know the reason.
Both of them are 'Main verb', not 'Auxiliary verb'. I mean the verb 'Do'.

Answer

As Ana says, modal verbs (can, could, must, may, might, will, would, shall, should) are followed by the base form of the verb. The main verb is not conjugated when it follows one of these verbs.

Also note that modal verbs don't have a 'to' either before or after them. The 'name' of the modal verb is simply 'can'.  The verb 'to can' has a different meaning - this is what is done when products are put into cans, as in 'canned fruit'.

Answer

After a modal verb like "to can" you must put the infinitive form of the verb without "to", not the present tense. "He can do everything". "He must do everything".