Check that the main verb following a modal verb is in its base form.

Ensure there is only one modal verb per clause.

Primary auxiliaries primarily indicate tense and voice.

Modal auxiliaries express mood, attitude, or nuance.

Primary auxiliaries can function as the main verb in a sentence.

Modal auxiliaries cannot function as the main verb.

Primary auxiliaries have different forms for tense and subject agreement.

Modal auxiliaries do not change form.

Primary auxiliaries can be combined with each other.

Modal auxiliaries cannot be combined with another modal.

Verbs that express modality and do not change form.

Always followed by the base form of the main verb.

Two modal verbs cannot be used together in a single clause.

Ability: can, could

Permission: can, may, could

Possibility: might, may, could

Obligation/Necessity: must, should, ought to, have to

Future/Prediction: will, shall

Verbs that can also function as main verbs and are used to help indicate tense.

Forms: is, am, are (present); was, were (past); been (past participle).

Uses: Forming continuous tenses; forming passive voice; expressing a formal plan or command; describing a state.

Forms: has, have (present); had (past); had (past participle).

Uses: Forming perfect tenses; forming perfect passive voice; expressing necessity (have to); discussing consumption or experience.

Forms: do, does (present); did (past); done (past participle).

Uses: Forming questions and negatives in simple tenses; adding emphasis; avoiding repetition in short answers.

Verbs used alongside a main verb to provide information about its tense, mood, or voice.

Indicate tense (past, present, future).

Express grammatical aspect (continuous, perfect).

Show modality (ability, permission, obligation).

Form the passive voice.

Add emphasis to a sentence.