A little while ago I was scrolling through my friend's Instagram & it seemed to me that one of her photos' description could be grammatically incorrect a bit.

The description is

"Today I saw the person who taught me to speak English..."

Right away I realized that I would go with "a person" not "the person" in that case.

(Quick facts: we are both Russians, and we don't have the articles in our language, so it's more or less expected that we might have some problems when it comes to put an article in the sentence.)

So, she writes about a friend (a person) of her. She does it the very first time in the context. Thus it seems pretty obvious to me to put "a" over "the".

On the other hand, theoretically the person the author talks about might be the only person in the author's life that was successful to teach the author to speak English. Is it then correct to go with 'the' in that case?

And last thing, I know that, for example, programmers and their managers use 'the user' all the times. They, as I suggest, just want to emphasize the importance of their users to them. According to that, I still think that if she wanted to emphasize the importance of the person she talked about, then she had to go with something like "Today I saw that person who taught me..."

P.S. To be honest, even though I think that 'a' is the correct one here, I'm pretty sure that an English teacher of mine would treat my 'a' as a mistake.

A little while ago I was scrolling through my friend's Instagram & it seemed to me that one of her photos' description could be grammatically incorrect a bit.

The description is

"Today I saw the person who taught me to speak English..."

Right away I realized that I would go with "a person" not "the person" in that case.

(Quick facts: we are both Russians, and we don't have the articles in our language, so it's more or less expected that we might have some problems when it comes to put an article in the sentence.)

So, she writes about a friend (a person) of her. She does it the very first time in the context. Thus it seems pretty obvious to me to put "a" over "the".

On the other hand, theoretically the person the author talks about might be the only person in the author's life that was successful to teach the author to speak English. Is it then correct to go with 'the' in that case?

And last thing, I know that, for example, programmers and their managers use 'the user' all the times. They, as I suggest, just want to emphasize the importance of their users to them. According to that, I still think that if she wanted to emphasize the importance of the person she talked about, then she had to go with something like "Today I saw that person who taught me..."

P.S. To be honest, even though I think that 'a' is the correct one here, I'm pretty sure that an English teacher of mine would treat my 'a' as a mistake.

Since it's not 100% clear for me I'm adding some:

1. I know the author in the real life. She studied English at school, she's doing it at university. So it's obvious there have been persons who taught her English. According to that and the James K's clarification he left under

I saw a person that taught me English

I still do guess that she was wrong & she had to use 'a'. So, she saw one of that persons that taught her.

2. She (author) wanted to express that the person she talked about was the only one from whom she really learnt how to speak English. The thing is in Russian we have two words учить and научить. Unfortunately, as far as I know, they both are translated to English as 'to teach', while the second one (научить) implies that the person you taught has completely understood what you taught one, i.e. the person completely learnt the lesson. And if she meant that, she had to express it in other way, not the way she did it, because it does not express what she really meant.