How do you choose the correct verb?

It depends on what action is happening and if it is something that is in the past present or future.

For example: My dog ate the cake yesterday.

The verb in this sentence is ate. Ate is the past form of eat. We use this verb because this is what the dog did. He didn't see the cake, write the cake, walk the cake.

Ate is in the past tense because it happened yesterday = the past.

If the action is happening now we would say:

My dog is eating the cake.

We use is because this verb shows use present/now. We use eating because this verb shows the action is happening over a period of time.

So it depends on the action and when it occurs.