We use already with the Present Perfect in affirmative sentences. We put it before the past participle or at the end of the sentence. Teddy has already left. / Teddy has left already. We use yet with the Present Perfect in negative sentences and questions. We usually put it at the end of the sentence. Nick hasn’t left yet. Has Nick left yet?We use just with the Present Perfect in affirmative sentences and questions to mean “a very short time ago”I’ve just finished my homework. Have you just eaten?
We use Present Perfect with time references such as this morning, today and this week to talk about the whole of that period of time up to now. I haven't seen Fred this morning. (It is still the morning.)Has Laura phoned today? (It is still the daytime.)We use Present Perfect to emphasis on number. She’s written three letters since this morning. She has spoken to two clients since 12 o’clock.
PAST SIMPLEFor actions that happened at a specific point in the past, usually with a time expression. Maria went to Spain last year. When? Last year. She called an hour ago,When? An hour ago. For past actions which happened immediately one after the other. She sealed the letter, put a stamp on it and posted it. For situations that happened in a period of time that has ended. For action which happened at a definite past time. The action was completed in the past. I lived in Kyiv from 2002 to 2014. (It’s 2021 now.)Shakespeare wrote a lot of plays. (Shakespeare is now dead; he won’t write again.)
PAST SIMPLEWith past time references, such as yesterday, last...and ... ago. We finished our exams last week. Karen started her new school a few days ago. With time references such as this morning, today and this week to talk about something that happened in a part of that period that is finished. I saw Fred this morning.(The morning is finished.) Did Laura phone today?(It is now the evening, so the main part of 'today' is finished.) With the question form How long ago ... ?How long ago did Tom break his leg? He broke it two years ago.
Note that it is possible to use how long with a past simple verb and for, but we can't use since in the past simple answer. How long did Tom have the rash for? (Tom no longer has the rash.)He had it for a month. NOT He had it since February. We can use the past simple or the present perfect with time expressions, such as: today, this morning, this evening, this week this year, etc. Compare the sentences: I wrote three emails this morning. (The speaker is talking in the evening, so the period of time has ended.)I've written three emails this morning. (The speaker is talking in the morning, so the period of time continues.)