Questions & Answers about Cuando la oruga coma la hoja, va a crecer.
Why do we use the subjunctive coma here instead of the indicative come?
Because of the word cuando. When cuando introduces an action that has not happened yet (the future), it forces the verb into the subjunctive mood. If we were talking about a daily habit ("Whenever the caterpillar eats the leaf..."), we would use the indicative come.
Can I use a future tense right after cuando, like cuando la oruga comerá?
No, Spanish never uses the future tense directly after cuando. To talk about the future after time words like cuando, you must use the present subjunctive. That is exactly why we use coma in this sentence.
If I wanted to say "When the caterpillar ate the leaf, it grew", would I still use the subjunctive?
No. The subjunctive is only used after cuando for future or anticipated events. For actions that are completed and in the past, you use the indicative mood. You would say Cuando la oruga comió la hoja, creció.
Why do we need the word a in va a crecer?
To say something "is going to" happen in Spanish, you use a conjugated form of the verb ir (to go), plus the preposition a, plus an infinitive verb. Here, va (it goes) + a + crecer (to grow) forms the future action.