Modal Verbs: could

Modal Verbs: could

Could can be used with the indefinite infinitive to express past actions. When using could in this way, it usually indicates ability or possibility in the past.

Modal Verbs: must

Modal Verbs: must

Must with the indefinite infinitive is used in the present to express a strong obligation, necessity or deduction.   Examples: I must study for my exam. (strong obligation)  

Auxiliary Verbs: to be

Auxiliary Verbs: to be

To be is used with the present participle (ing form) of the main verb to indicate an action that is currently happening.   Example: She is studying for her exam.

The Future Perfect Continuous in the Past

The Future Perfect Continuous in the Past

The Future Perfect Continuous in the Past is a tense used to talk about an ongoing action that was expected to be completed at a specific point in the past. It is formed using the past participle of will have been (which is would have been) and the present participle form (-ing) of the main verb.