I need help. I am sorry if my English is bad and the question easy, but bear with me, I am only a 16 years-old foreigner.

I read 'Present Simple versus Present Perfect' thread a few minutes ago. I admit that the person who explained it did it wonderfully, but I still have one question. It was said that, being the Present Tense, Present Perfect can be used in sentences with 'now'.

e.g. I have written a letter now.

and I've seen lots of people using it with 'just'

e.g. I have just finished my assignment.

Can I use Present Simple in those sentences? If I can, what is the difference? Please explain it to me a bit.

Thank you.
Hello Astraea

I too am a learner of English, but let me answer to your question.

Are you asking if the sentences like below are correct?
I write a letter now.
I just finish my assignment.
No, both are incorrect. Do you ask why? Are you asking why? It's because "write" and "finish" are dynamic verbs. You can say "I live now in Zagreb", because "live" is a stative verb. But in the case when the verb is dynamic, the simple present tense can be used only when you talk about some habitual activity. For example,
I write a letter to my mother once a week.
Every week I finish my assignment before Friday night.

If you want to say some one-time event using a dynamic verb, you have to say it in [1] the past tense, [2] the present perfect tense, [3] the present progressive tense, [4] or the future tense.
[1] I wrote a letter to my mother yesterday.
[2] I have written a letter to my mother now.
[3] I am now writing a letter to my mother.
[4] I will write a letter to my mother tomorrow.

Hi paco,

It's a superb job you did above to explain Astraea's question...I learned a lot myself!

But what about the sentence you used above: Do you ask why?
I think it sounds better if you say: Are you asking why? or Do you want to know why?

Do you agree?
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Hi Danyoo

Thank you for pointiong out my wrong collocation. As you know well, I'm still a mere learner of English. So I really appreciate such help.

I know you are an expert in grammar, but putting into practice what you already know can be tricky for a non-native speaker. I need to be very deliberate in certain things myself, as to avoid making silly mistakes. Hey, but as long as we both continue to learn, that's the most important thing!

Emotion: smile
Thank you for explaining, paco. I still don't know if I could say 'Like I just said'. or 'I wrore a letter now.' The last one sounds a bit awkward to me...

It's funny that you mentioned Zagreb, I'm from Croatia. Emotion: smile
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Hello Astraeal

"Just" has many meanings as you can see below:
1. merely, simply, only -- (to say there is nothing more than that)
I was just (=merely) asking./ It is just (=simply) a matter of time./ He was just (=only) a child.
2. precisely, exactly, truly -- (to indicate exactness or preciseness)
He was doing just (=exactly) what she had told him to do./ It was just (=exactly) as he said--the jewel was gone.
3. just now -- (only a moment ago)
He has just arrived = He arrived a moment ago./ The sun just now came out = The sun came out a moment ago.
4. simply, indeed -- (to intensify absoluteness)
I just (=simply) can't take it anymore./ He was just (=simply) grand as Romeo./ It's just (= simply) beautiful!

"I wrote a letter now" sounds odd to me, though "I wrote a letter just now (=a moment ago)" is fine. "Like I just said" is not "like I said a moment ago" but it is "like I exactly said". (EX) "Like I just said, I have two sons and a daughter". Native speakers seem to use "like I just said" even when they did not say anything previously about the topic they are going to speak. It may be a sort of nonsense filler to smooth their speech just like "as you know,".