Which of the following sentences contains a modal verb?

The sentence "I can swim very well" contains the modal verb "can." Modal verbs are auxiliary verbs used to express possibility, ability, permission, or necessity. In this case, "can" indicates the ability to swim, suggesting that the speaker is capable of swimming well. Modal verbs are characterized by their unique function; they do not change form based on the subject and cannot be used alone as main verbs.

The other sentences do not include modal verbs. "I swim every day" uses the simple present tense to state a routine activity, while "I swam yesterday" employs the simple past tense to refer to a specific past action. "I will swim tomorrow" includes the word "will," which is a future auxiliary verb, but grammatically, this construction does not qualify as a modal verb in its standalone form. Thus, the presence of "can" in the correct sentence makes it the only option that contains a modal verb.