Learn English – Past tense question: weren’t vs. wouldn’t

Context, context, context!

Briefly: the simple past is used to narrate past events. The present perfect is used to mention past events which give rise to a present state which is of present interest.

The children played in the sandbox for a while, then moved to the swings. At four o'clock their mother called them inside because it looked like rain.

The children have played in the sandbox and left their toys there. Tell them to go bring their toys inside.

However, the second could also be expressed with a simple past, and would probably be expressed with a past progressive. Play is an activity verb, which doesn't accommodate the perfect all that well. This example, with a telic verb (one which has a distinct change of state in its meaning) may be more instructive.

The children finished their chores, then went outside to play until their mother called them in.

The children have finished their chores; I am going to let them go outside and play now.

I'm going to change the verb in the sentence:

1. I didn't eat.

2. I haven't eaten.

Both mean the person did not consume a meal during some timeframe. As others have said, the tenses are different.

Did you eat yesterday?
No, I didn't eat yesterday.
_{(In this case, the simple past maps to yesterday)}

When was the last time you ate?
I haven't eaten since last Friday.
_{(In this case, the present perfect refers to a time period starting last Friday and continues up to now.)}

Similarly:

Did you go to the see the new movie last Friday?
No, I didn't go on Friday, I went on Saturday.

Have you gone to see the new movie yet?
No, I haven't gone yet. I'm planning to go this weekend.

When using the first person, I think we usually use go instead of come in conversations like these. However, we might say:

Did he come to the family reunion last week?
No, he didn't come. He had to work that day.

Has he come to any of the family reunions lately?
No, he hasn't come to a reunion since 2007.