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I'm always confused when to use past tense and perfect present tense. They're all have the same purpose: they define the action whether it is near past or distant past. So when will I use these tenses, anyway?

Example:
I read an article that banana makes you happy.
I have read an article that banana makes you happy.

If they are both correct, could you tell me what's their differences.

For the preposition:

I was wondering about this sentence:
I slept in my bed

Don't you think it should be "I slept on my bed because I was sleeping at the top of the bed, and not inside the bed. I hope you get what I'm trying to say.
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The first sentence is not in past simple tense yet. Past simple points to a particular time in the past. The sentence should be:

I read an article yesterday that banana makes you happy.

The second sentence is in present perfect. You aren't specifying when the article was read, but in the above revised sentence, you do.

Just to clarify, the following sentence is incorrect:

I have read an article yesterday that banana makes you happy.

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I love prepositions! sarcasm

But you are right. It should be "slept on", but let's look at those sentences carefully.

I slept on my bed last night.
I slept in my bed last night.

The "on" preposition indicates your position on a surface. Both sentences sound correct, but the second one is informal. However, to the native speaker during informal conversations, it would sound correct.

Dave: Tom, where did you sleep last night?
Tom: I slept in my bed.

Now, consider the following examples:

I slept on the table last night.
I slept in the table last night.

The second sentence clearly sounds odd. You can't sleep in the table! Emotion: stick out tongue
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Hi,
1. I read an article that says eating a banana makes you happy. -- That is a simple fact. Here, you are merely
telling a story of what happened in the past. Note that even though the time is not specified, this statement
is still written in the past tense.

2. I have read an article that says eating a banana makes you happy. -- Your reading of this article is of
current relevance; it is somehow related to the present situation. Here, you are giving a description of an
event that took place in the past, but still has a clear connection with the present situation.

Adding 'eating' to those sentences sounds more reasonable to me.

I slept in my bed. -- That sentence implies 'under the covers'.
I slept on my bed. -- That sentence implies 'on top of the covers', just as you suggest.

Regards
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Comments  
You you use the past tense like simple past if something happened in the past and has finished and if this action had no consequences in the prsent. And the present perfect tense: if the action happened in the past and had not finished or had consequences in the present.
Emotion: big smile
 Regards's reply was promoted to an answer.
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One more thing before I let this thing dies, what's the right preposition in this sentence: I keep (on/in) searching, and searching until I find it. Answer please....

Anyhow, what's the difference between "in" and "on" when I'm stating an action, just like the example I've given to you.