Question

Quando usamos os modal verbs: CAN, MAY, e ON ??

Answer 1

Modal verbs:
.Come before the subject in questions.
.Do not change their form according to the tense; there is no "s" added to the verb (in 3 person).
.Take infinitives without "to" for their complement.
.Take "not" after the verb to make a verb negative.
MUST is used to express strong subjective obligation or necessity.

Example: I MUST leave now.

HAVE TO (not exactly a modal verb) is used to express objective obligation, imposed by the external power (scholl, government, police, etc.)

Example: All Brazilians over 18 and under 70 years of age HAVE TO vote in Brazil.

MAY and MIGHT are used to express possibility. MIGHT is less formal than MAY, and more common in conversation.

Example: Melissa MIGHT not come to the party. / Jack MAY, win the tennis tournament.

CAN'T is used to express a prohibition.

Example: Students CAN'T leave the classroom during the test.

Answer 2

*Can - Usado nas habilidades físicas e mentas (capacidade de fazer algo).

Example: Can you drive to me ? (você pode dirigir para mim?)

**May - Usado para pedir autorização ou permissão para fazer algo.

Example: May I help you? (Posso te ajudar?)

***On - Usado quando algo está anexado ou complementando algo.

Examples: 
a pain on her knee. (uma dor no joelho dele)
I go to the beach on Sunday. (eu vou à praia no domingo)
On Brazil (No Brasil)