Modals are the 'attitude' words of English. Without them, you can only state raw facts like 'I swim'. Modals let you express ability ('I can swim'), obligation ('I must swim'), or advice ('I should swim').
A modal verb is a helper verb that expresses necessity or possibility. Unlike normal verbs, modals don't change their form (they never take an '-s' or '-ed') and are always followed directly by the base form of a main verb without 'to'.
A very common mistake is adding 'to' after a modal verb (e.g., 'I must to go'). This is incorrect! Always use the bare infinitive: 'I must go'. Another trap is trying to add '-s' for he/she/it: 'He cans swim' is wrong; it's simply 'He can swim'.
Change the modal verb to see how the meaning of 'I [modal] study' changes.
Test your understanding of modal verbs!
Fun fact: 'Will' and 'Would' are also modal verbs! This means the future tense in English ('I will go') is actually just using a modal verb to express certainty about a future event.