confusion

Iv been confused by this for a long time.

should I say:

I was with her and she thought I was an English but actually I was a French.

OR

I was with her and she thought I was an English but actually I am a French.

in a past tense sentence, if Id like to address something my characteristics, like my name, my gender, my nationality, should I follow the tense iv been using in the sentence or I switch?

like

two days ago I talked with a girl and she seemed knew me and asked my family, but I wasn't that John guy, but finally I managed to let her knew that I WAS CLEVELAND (I AM CLEVELAND) not John.

I'd strongly prefer the present in the first sentence, although some people might say it with the past tense. (And sometimes people make it a joke. "But actually I was French. And I still am!") Sometimes the past tense is appropriate, though. If you were referring to a third person instead of yourself, using the past tense is would be more likely, especially if you're talking about somebody you don't know, in the same way that we say "Who was that?" rather than "Who is that?" after the person has already left.

In the "I was/am Cleveland" example, I would definitely say "I am Cleveland". The reason is that, until you said that, she still thought you were John. At the time, your identity was still under question in the present. There are contexts where "I was Cleveland" might make sense, but I think you can't go wrong with "I am Cleveland" (unless you're no longer Cleveland ;)).

By the way, I wouldn't use an article with "English" and "French" in those sentences. You could just leave them out (turning them into adjectives), or you could say "Englishman" and "Frenchman" instead.

- Kef

the reason why this whole thing has been confusing me for such a long time is that when I first learned English, I remember there is a fixed way to use English "all tenses in one sentence have to be unified", but now I found that it's hard to unify tenses even in a very short sentence. Every time when I talk to somebody and encounter this problem I will have a pause and not sure how to make it correctly.

All tenses in a sentence do NOT have to be unified, and I've heard that mistake made by non-natives before; to us it sounds odd, but it's always hard to explain to them why it's wrong.

God! my teachers had kept telling me about this since junior high, then senior high, then university...hmm...they are evil...(by their point that here should be "they were evil")

so to make this sentence 100% NOT odd, should I say

two days ago I talked with a girl and she seemed knew me and asked my family, but I am not that John guy, but finally I managed to let her knew that I am Cleveland not John.

or should I use "I wasn't that John guy"?

Shouldn't it be ... I managed to let her KNOW that...

* she seemed to know me

Well, Cleveland, your sentence is at least 50% odd anyway, because there are multiple grammatical mistakes in it. Instead of this:

two days ago I talked with a girl and she seemed knew me and asked my family, but I am not that John guy, but finally I managed to let her knew that I am Cleveland not John.

try this:

Two days ago I was talking to a girl who seemed to know me and asked about my family, but she had me confused with another guy named John. I finally managed to let her know that I was Cleveland, not John.