Grammar Rule: Sequencing Past Events

The original sentence contains two past actions: going to the cinema and the film starting. To correctly express these events in sequence, the action that happened first requires the past perfect tense.

The structure for past perfect tense is $had$ + past participle.

• The action of going to the cinema ("When we went to the cinema") is in the simple past tense, indicating the later event.
• The action of the film starting happened *before* we arrived. Therefore, it should be in the past perfect tense.

The past participle of the verb 'start' is 'started'. Applying the past perfect structure gives us $had started$.

Analyzing Substitution Options

• 1. $had already started$: This correctly uses the past perfect tense, indicating the film's start occurred before arriving at the cinema.
• 2. $has started$: This is the present perfect tense. It's incorrect because both actions occurred in the past.
• 3. $would already start$: This form suggests a conditional or habitual past action, not a completed event prior to another past event.
• 4. $was already start$: This option is grammatically incorrect. The correct past continuous form would be 'was starting', which implies the action was in progress, not completed beforehand.

The most appropriate substitution to maintain the correct sequence of past actions is $had already started$.