Mikkel...
Hailey's Hayley's answer is correct.
The simple past tense always refers to an action or situation that is finished.
I would not use the present perfect tense unless the sentence included one of the 'time words/markers/expressions'.
You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc.
We CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, for (a period of time), and just.
So...... simple past tense only
Hi Mikkel, great question!
A native speaker would notice that, actually.
"I saw him yesterday" works great, as Hayley had pointed out.
Another correction, "alright" instead of "all right" ;)
@Mikkel, I agree with your friend that English is easy to learn but difficult to master:) One important reason, I think is the variation in grammar, vocabulary and structure according to the geographical location. Generally, we consider US-English and British-English as the standard English but as we are considering two references we get confused by listening different structures indicating same meaning!
English is now influencing the people belongs to different cultures and community so they are speaking or writing English sometimes by looking at US-English and sometimes by looking at British-English. This is where they put their legs in two boats!!!
Except all this, English is a dynamic language and I remember a discussion shared by @Alan regarding how English will sound in next 100 year!!
Here is a link I came across and it's about how will the future of English be like.