אני מכירה את הזוג הזה, כי הם גרים באותו בניין.

Because the speaker is female.

In the present tense, Hebrew verbs like מכיר / מכירה agree with the subject’s gender and number:

• אני מכיר = I know / am acquainted with... (said by a male speaker)
• אני מכירה = I know / am acquainted with... (said by a female speaker)

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

• אני מכיר את הזוג הזה...

Not exactly. In the present tense, Hebrew usually shows gender and number, but not person.

So מכירה by itself could mean:

• I know (if said by a woman)
• you know (to one woman)
• she knows

Because of that, Hebrew often uses the pronoun to make the subject clear:

• אני מכירה = I know
• היא מכירה = she knows
• את מכירה = you know (to a woman)

So אני is helpful and usually necessary unless the context already makes it obvious.

Hebrew usually uses להכיר for being acquainted with people, places, or things.

• אני מכירה את הזוג הזה = I know this couple / I’m acquainted with this couple

But לדעת is usually used for facts, information, or how to do something:

• אני יודעת את התשובה = I know the answer
• אני יודעת עברית = I know Hebrew
• אני יודעת לבשל = I know how to cook

So when talking about knowing people, להכיר is the normal verb.

את is the direct object marker. It does not get translated into English.

Hebrew uses את before a definite direct object. A noun is definite if it is something like:

• marked with ה־
• a proper name
• a pronoun
• a noun with this/that
• something otherwise clearly specific

Here, הזוג הזה = this couple, which is definite, so Hebrew uses את:

• אני מכירה את הזוג הזה

Compare:

• אני מכירה זוג = I know a couple
• אני מכירה את הזוג הזה = I know this couple

So את is there because the object is specific and definite.

Because in Hebrew, demonstratives like this usually come after the noun:

• הספר הזה = this book
• האישה הזאת = this woman
• הילדים האלה = these children
• הזוג הזה = this couple

So the normal pattern is:

• the + noun + this/that

Also, the noun is usually definite when you use this/that, which is why you get הזוג.

By contrast, זה הזוג usually means something more like:

• This is the couple

So:

• הזוג הזה = this couple
• זה הזוג = this is the couple

זוג is grammatically singular and masculine.

That is why the phrase is:

• הזוג הזה = this couple

not a plural form.

But a couple refers to two people, so when Hebrew continues and talks about the people themselves, it often switches to plural:

• כי הם גרים... = because they live...

So the sentence treats הזוג as a single unit at first, and then refers to the two individuals as they.

You could also hear:

• הזוג הזה גר באותו בניין

That would treat the couple more as one unit: This couple lives in the same building.

But הם גרים is very natural because it focuses on the two people.

הם גרים is the masculine plural form.

In Hebrew, masculine plural is used for:

• groups of males
• mixed-gender groups
• groups whose gender is unspecified or not important

Since זוג often refers to a couple in a general way, הם גרים is the normal default.

If you were clearly talking about two women, you could say:

• הן גרות

So the choice depends on who the couple is, but הם גרים is the standard general form.

גרים is the present tense masculine plural form of לגור, which means to live / reside.

Forms of לגור in the present tense:

• גר = he lives
• גרה = she lives
• גרים = they live (masculine / mixed)
• גרות = they live (feminine)

So:

• הם גרים = they live

This verb means to live somewhere / reside, not to live in the broader sense of being alive.

Here באותו means in the same.

It is made of:

• ב־ = in
• אותו = same

So:

• באותו בניין = in the same building

This אותו is different from the object pronoun אותו meaning him/it, even though it is spelled the same in unpointed Hebrew.

Here you can tell it means same because it is followed by a noun:

• אותו בניין = the same building

It also agrees with the noun:

• באותו בניין = in the same building (masculine singular)
• באותה דירה = in the same apartment (feminine singular)
• באותם בתים = in the same houses (masculine plural)
• באותן ערים = in the same cities (feminine plural)

Because אותו in the sense of same already makes the phrase definite.

So Hebrew says:

• באותו בניין = in the same building

not:

• באותו הבניין

This is normal with אותו / אותה / אותם / אותן meaning same.

So even though English says the same building, Hebrew does not usually put ה־ on the noun in this structure.

Yes. That would also be correct.

There is a small difference in feel:

• כי הם גרים באותו בניין focuses on the two people: because they live in the same building
• כי הזוג הזה גר באותו בניין treats the couple more as a single unit: because this couple lives in the same building

Both are grammatical. The version with הם גרים sounds very natural because a couple is made of two people, so speakers often switch to plural when talking about what the couple members do.