I have lost my keys, so I can’t get in the house.

[Alice Chu]

Please tell me if my understanding is correct.
1. I have lost my keys, so I can’t get in the house.
The present perfect (have lost) indicates that something happened in the past and has an effect in the present.

2. I lost my keys last night, so I couldn’t get in the house.
The past simple (lost) indicates that something happened in the past and has nothing to do with the present.

Are the following sentences correct?
3. I lost my keys, so I can’t get in the house.
4. I lost my keys, so I couldn’t get in the house.

 

[5jj]

Please tell me if my understanding is correct.
1. I have lost my keys, so I can’t get in the house.
The present perfect (have lost) indicates that something happened in the past and has an effect in the present.

That's partly true,. However, as we have told you more than once, it's the full context rather than exclusively the tense/aspect, that tells us what the message is.

2. I lost my keys last night, so I couldn’t get in the house.
The past simple (lost) indicates that something happened in the past and has nothing to do with the present.

Not necessarily.

Are the following sentences correct?
3. I lost my keys, so I can’t get in the house.
4. I lost my keys, so I couldn’t get in the house.

Both are grammatically possible.

 

[jutfrank]

The present perfect (have lost) indicates that something happened in the past and has an effect in the present.

We usually call the relation between the past and present 'relevance'. It's not exactly an 'effect' in the present, necessarily.

2. I lost my keys last night, so I couldn’t get in the house.
The past simple (lost) indicates that something happened in the past and has nothing to do with the present.

Be careful here. As 5jj I think suggests above, the use of past simple doesn't tell us whether the past action has anything 'to do' with the present in the speaker's mind. What I think you're trying to say is that the past simple aspect does not suppose any relation between past and present in itself. Make sure you distinguish carefully between linguistic meaning (what the language is literally saying) and the wider meaning (what is meant and understood by the utterance as a whole).

 

[Tdol]

With #3, I would use it if you had an idea when you lost them- I lost my keys this morning and can't get into my house. I would not use it when standing outside my house going through my pockets for the third time.