Learn Hardcore Chinese: Wǒ yǒu yì gè nǚ tóngxué, tā bǐ wǒ dà sān suì. - I have a female classmate who is three years older than me.

女同学 (nǚ tóngxué) explicitly tells you the classmate is female.
她 (tā) is just the usual 3rd-person singular pronoun “she / he” in spoken Chinese (它 only written distinguishes gender; spoken tā sounds the same for 他 / 她).

Using 女同学 sets up who we’re talking about: “a female classmate of mine.”
Then 她 is the natural pronoun to refer back to that person.

You could say:

我有一个女同学，她比我大三岁。 – I have a female classmate; she is three years older than me.
我有一个同学，他比我大三岁。 – I have a classmate; he is three years older than me.

If gender isn’t important, you can simply say:

我有一个同学，比我大三岁。 – I have a classmate who is three years older than me. (subject of the second clause is understood)

How does 比 work in 她比我大三岁? Where is the “than” and where is “older”?

比 (bǐ) marks a comparison. Basic pattern:

A 比 B + adjective (+ degree / amount)
A is more [adjective] than B (by X)

In this sentence:

她 – she (A)
比我 – compared to me, than me (B)
大三岁 – older by three years (adjective + amount)

So:

她比我大三岁。
Tā bǐ wǒ dà sān suì.
Literally: She, compared to me, is big by three years.
Natural English: She is three years older than me.

More examples:

今天比昨天冷。 – Today is colder than yesterday.
我比他高一点儿。 – I’m a bit taller than him.
这本书比那本贵很多。 – This book is much more expensive than that one.

Why is 岁 at the end: 比我大三岁? Could I say 比我三岁大?

The natural order for this pattern is:

比 + somebody + 大 + number + 岁

So:

她比我大三岁。 – She is three years older than me.

Here, 大 is the adjective (big/old) and 三岁 is the “by how much” part.

比我三岁大 is not the usual structure and sounds wrong in modern Mandarin. The age difference expression almost always follows the pattern:

大 + number + 岁 (older by X years)
小 + number + 岁 (younger by X years)

For example:

我比他大两岁。 – I’m two years older than him.
他比我小一岁。 – He’s one year younger than me.

Can I say 她大我三岁 instead of 她比我大三岁? Is there any difference?

Yes, 她大我三岁 is also correct and quite natural.

她比我大三岁。
她大我三岁。

Both mean: She is three years older than me.

Difference in structure:

她比我大三岁 uses the standard comparison pattern with 比.
她大我三岁 uses A + 大 + B + number + 岁 (“She is older than me by three years”) without 比.

Both are common. For learners, it’s usually easier to master the 比 pattern first.

Why is there no 是 in 她比我大三岁? Why not 她是比我大三岁?

In this comparative structure, 是 (shì) is not needed and usually incorrect.

The core predicate is already:

比我大三岁 – is older than me by three years

Adding 是 would be like saying “She is is three years older than me.”

是 is mainly used for:

Noun = noun:
- 我是学生。 – I am a student.
Identification / emphasis (with 的 / 是…的 structure):
- 是他帮了我。 – It was he who helped me.

For plain adjective or comparative sentences, you normally do not use 是:

他很高。 (not 他是很高) – He is tall.
今天比昨天冷。 (not 今天是比昨天冷) – Today is colder than yesterday.
她比我大三岁。 (not 她是比我大三岁)

Can the two clauses be combined, like 我有一个比我大三岁的女同学? Is that more natural?

Yes, that combined version is very natural and common:

我有一个比我大三岁的女同学。
I have a female classmate who is three years older than me.

Here, 比我大三岁的 acts like a relative clause modifying 女同学 (“a female classmate who is three years older than me”).

Both versions are fine:

我有一个女同学，她比我大三岁。 – Two separate clauses.
我有一个比我大三岁的女同学。 – One clause with a modifier.

Spoken Chinese uses both. Version (2) sounds slightly more compact and descriptive.

Why do we say 一个女同学 instead of just 有个女同学? Can 一 be omitted?

Both forms are possible:

我有一个女同学。 – I have a female classmate.
我有个女同学。 – I have a (certain) female classmate.

In colloquial speech, 一 (yí) is often dropped when the number is one, especially in 有 + (一)个:

我有个朋友。 – I have a friend.
我有个老师。 – I have a teacher.

Keeping 一 (一个) can sound a bit more careful / emphatic on “one,” but in this context there’s no big difference in meaning. Pronunciation-wise, even when written 一个, it’s usually pronounced yí ge, not yì ge in natural speech.

Why use 岁 to talk about age? Can I use 年 instead, like 三年?

When you talk about age, you almost always use 岁 (suì), which is a special measure word for years of age.

我二十岁。 – I’m 20 years old.
她比我大三岁。 – She is three years older than me.

年 (nián) means years (in time), not age:

三年 – three years (duration)
- 我在中国住了三年。 – I lived in China for three years.

So you should not say ✗ 她比我大三年 when talking about age; the natural form is 她比我大三岁.

Why 女同学 and not 女学生? What’s the nuance of 同学?

同学 (tóngxué) literally means “same study” → classmate / schoolmate.
It focuses on the relationship between you and the person: someone who studies at the same school or in the same class.
学生 (xuésheng) simply means student and does not say anything about their relationship to you.

So:

女同学 – a female classmate (someone you study with).
女学生 – a female student (from the teacher’s or outsider’s perspective).

In the original sentence, you want to say “my (female) classmate”, so 女同学 is the natural choice.

In English we’d probably say “an older female classmate,” not “a female classmate, she is older by three years.” Is it also natural to say “older female classmate” in Chinese?

Yes, you can fold the “older by three years” part into the noun phrase using a modifier:

我有一个比我大三岁的女同学。
I have a female classmate who is three years older than me.
(Very natural.)

You could also be less specific:

我有一个比我大的女同学。 – I have a female classmate who’s older than me.

Chinese often uses adjective / phrase + 的 + noun to pack descriptive information before the noun, similar to “older female classmate” or “a female classmate who is older than me.”

How is 女 (nǚ) pronounced? It seems hard for English speakers.

女 (nǚ) has:

Initial: n- (as in no).
Final: -ü (the “u with two dots”), plus third tone: nǚ.

To get ü:

Say “ee” as in “see”.
Without moving your tongue, round your lips as if you’re saying “oo”.
That tight, front, rounded vowel is ü.

Practice pairs:

女 (nǚ) vs 奴 (nú) – female vs slave
女 (nǚ) vs 怒 (nù) – female vs anger

The tone is third tone: start mid, dip down, then rise a bit: nǚ.

In English, “older” is an adjective that doesn’t mention “big.” Why does Chinese use 大 (big) to mean “older” in 大三岁?

In Chinese, 大 (dà) also means “older” when talking about age.

大 – big / older
小 – small / younger

Common age-related uses:

他比我大。 – He is older than me.
她比我小两岁。 – She is two years younger than me.
大哥 – eldest older brother
小妹妹 – little younger sister

So 大三岁 literally means “big(ger) by three years,” i.e. three years older.

Could we swap the order to 三岁比我大 or something similar?

No. That would be ungrammatical.

The standard, natural pattern is:

(Subject) + 比 + (reference) + 大/小 + number + 岁

So:

她比我大三岁。 – She is three years older than me.
我比他小一岁。 – I am one year younger than him.

Putting 三岁 before 比我 (✗ 她三岁比我大) or after 比我 but before 大 (✗ 她比我三岁大) does not fit the normal grammar for age comparison in modern Mandarin.

Stick to:

她比我大三岁
or 她大我三岁 (without 比).