היא סיפרה לי עליו, אבל אני לא מכירה אותו.

Because the subject is היא (she). In Hebrew past tense, the verb agrees with the subject in person, number, and gender.

• היא סיפרה = she told
• הוא סיפר = he told

So סיפרה is the correct 3rd person feminine singular past form of לספר.

Because מכירה is feminine singular, so it shows that the speaker is female.

In the present tense, Hebrew uses forms that agree with the subject's gender and number:

• female speaker: אני מכירה
• male speaker: אני מכיר

So this sentence is being said by a woman. If a man were speaking, it would be:

היא סיפרה לי עליו, אבל אני לא מכיר אותו.

Hebrew usually uses two different verbs where English often just uses know:

• להכיר = to know/be acquainted with a person, place, or thing
• לדעת = to know a fact, information, or how to do something

Here the sentence means I don't know him in the sense of I am not acquainted with him, so מכירה אותו is the natural choice.

Compare:

• אני לא מכירה אותו = I don't know him personally
• אני לא יודעת עליו הרבה = I don't know much about him

עליו means about him here.

It is made from:

• על = on/about
• a pronoun ending meaning him

So:

• עליו = about him
• עליה = about her
• עליי / עלי = about me
• עליך = about you
• עלינו = about us

With the verb לספר, על usually means about:

• לספר על מישהו = to tell about someone

Because Hebrew says לספר ל־מישהו for to tell someone.

So:

• סיפרה לי = she told me
• סיפרה לו = she told him
• סיפרה להם = she told them

The ל preposition marks the person receiving the information.

English says told me, but Hebrew structure is closer to told to me.

Because the verb להכיר takes a direct object, not על.

So:

• אני לא מכירה אותו = I don't know him
• אני לא מכירה אותה = I don't know her

If you said אני לא מכירה עליו, that would not be the normal way to say I don't know him.

So the sentence changes from:

• סיפרה לי עליו = told me about him to
• לא מכירה אותו = don't know him

No. אותו already functions as the direct-object form him.

Compare:

• with a noun: אני מכירה את דני = I know Danny
• with a pronoun: אני מכירה אותו = I know him

So you do not say את אותו here.

This is a very common point for learners: when the object is a pronoun like אותו, אותה, אותם, you do not add a separate את before it.

It is natural and useful here.

In Hebrew present tense, the verb form מכירה shows gender and number, but not clearly person by itself in the way past/future verbs do. So the pronoun אני helps make the subject explicit.

Also, after אבל (but), אני adds a natural contrast:

• She told me about him, but I don't know him.

So אני sounds very normal here.

Yes. You would change the gendered forms as needed.

If the speaker is male:

• היא סיפרה לי עליו, אבל אני לא מכיר אותו.

If they are talking about a woman:

• היא סיפרה לי עליה, אבל אני לא מכירה אותה.

So pay attention to two different gender areas:

1. the speaker's gender in מכיר / מכירה
2. the other person's gender in עליו / עליה and אותו / אותה

Because the sentence has two clauses:

• היא סיפרה לי עליו
• אבל אני לא מכירה אותו

The comma helps separate them, just like in English before but. It is standard punctuation and makes the contrast clearer.