Learn Hardcore Chinese: Tā jīnnián èrshí suì, mèimei bǐ tā xiǎo liǎng suì. - He is twenty years old this year; his younger sister is two years younger than him.

Using 是 here (他今年是二十岁) is usually unnecessary or sounds slightly off in neutral speech. It’s only used in special contexts for emphasis or contrast, for example:

他去年十九岁，今年才是二十岁。
Last year he was 19; it’s only this year that he’s 20.

So for a simple statement of age, 他今年二十岁 (or just 他二十岁) is the normal form, without 是.

Could I say 他今年是二十岁吗？ as a question?

You can, but it has a specific feel.

他今年二十岁吗？
is the neutral way to ask: Is he 20 this year?
他今年是二十岁吗？
adds a sense of checking or contrasting, like: So this year he *is 20, right?* (implying you’re confirming information, or contrasting with some other age you had in mind).

For everyday use, when just asking about age, 他今年二十岁吗？ is the most natural.

Why are 今 and 年 written separately but glossed together as jīnnián? Is 今年 one word?

In real Chinese writing, 今年 is written without a space and is treated as one word meaning “this year.”

In your annotated sentence, you see:

今　年 {jīnnián}

This is just a teaching/annotation choice: the gloss shows the pronunciation jīnnián for the two characters together, because together they form the word 今年.

So:

今年 = one word, pronounced jīnnián, meaning this year.
Splitting it into 今 and 年 is just for character learning; you would not put a space there in normal writing.

Can I change the word order and say 今年他二十岁 instead of 他今年二十岁?

Yes, both are correct.

他今年二十岁。
今年他二十岁。

They are both grammatical and common. The difference is a slight change in emphasis:

他今年二十岁: starts with 他, lightly focusing on “he.”
今年他二十岁: starts with 今年, lightly focusing on “this year” as the topic.

In most everyday contexts, they’re interchangeable.

In 妹妹比他小两岁, how exactly does the 比 structure work?

比 (bǐ) is a comparison word. The basic pattern is:

A 比 B + Adjective (+ degree/difference)

In your sentence:

妹妹 = A (the one being compared)
他 = B (the reference point)
小 = adjective “small/young”
两岁 = the difference (two years)

So:

妹妹比他小两岁。
Literally: Younger sister compared to him is small by two years.
Natural English: His younger sister is two years younger than him.

Other examples with the same pattern:

他比我大三岁。 – He is three years older than me.
这本书比那本贵很多。 – This book is much more expensive than that one.

Why does 小 mean “younger” here? Could I use 年轻 instead?

In family and age comparisons, Chinese very often uses:

大 (big) = older
小 (small) = younger

So:

他比我大两岁。 – He is two years older than me.
妹妹比他小两岁。 – The younger sister is two years younger than him.

You can use 年轻 (niánqīng) to talk about being “young,” but:

年轻 is about being youthful in general.
大/小 with an age difference (大两岁 / 小两岁) is the standard, concise way to say someone is older or younger by a certain number of years.

So 妹妹比他小两岁 is the most natural expression here; 年轻 would not usually be used with a specific age difference like this.

Why is it 两岁 but 二十岁? When do I use 二 and when do I use 两?

Both 二 (èr) and 两 (liǎng) mean “two,” but they’re used differently.

General rule of thumb:

Use 二 inside numbers (20, 22, 200, etc.):
- 二十 (20)
- 二十二 (22)
- 二百 (200)
Use 两 before measure words / classifiers and many counting words:
- 两岁 – two years (of age)
- 两个学生 – two students
- 两本书 – two books

In your sentence:

二十岁: 20 years old → 20 is a number, so 二十.
两岁: two years (difference in age) → number + measure word (岁 is like a measure word for age), so 两岁, not 二岁.

So the combination is perfectly normal:

二十岁 (20 years old)
小两岁 (younger by two years)

Does 妹妹 automatically mean “his younger sister”? Why isn’t it 他的妹妹?

In this sentence, 妹妹 is understood to be his younger sister from context.

Chinese often drops possessive markers (的) when:

The relationship is clear from context, or
It’s a close family relationship or in-group relation.

So:

妹妹比他小两岁。
In context where we’re talking about “him,” it’s natural to understand 妹妹 as 他的妹妹.

If you need to be explicit (for clarity or because there could be confusion), you can say:

他的妹妹比他小两岁。

Both are grammatically correct; the short form 妹妹 just sounds more natural and less heavy when the context is obvious.

Can I say 妹妹小他两岁 instead of 妹妹比他小两岁?

Native speakers do sometimes say:

他大我三岁。
她小我两岁。

So 妹妹小他两岁 is not ungrammatical, but:

In standard textbook grammar, the 比-construction (妹妹比他小两岁) is the canonical pattern.
A 小/大 B + number + 岁 is more colloquial and slightly less neutral/formal.

For a learner, it’s safest to stick with:

妹妹比他小两岁。 – standard and very common.

Later, as you get more comfortable, you’ll hear and possibly use patterns like 小他两岁 / 大我三岁 in speech.

Is it okay to just say 他二十岁 without 今年?

Yes.

他二十岁。 – He is 20 years old.
他今年二十岁。 – He is 20 this year.

今年 adds the nuance “this year” (he becomes/turns 20 this year). Without 今年, it’s a simple statement of his current age. Both are natural; which you choose depends on whether “this year” is important to the context.

Could I say 他今年二十岁了? What does the 了 add?

Yes, 他今年二十岁了 is very natural.

了 here marks a change of state or a new situation.
他今年二十岁了。 implies something like:
- He’s 20 this year (he has now reached 20).
- There is a sense of transition, often with an underlying feeling like “He’s already 20 now.”

Compare:

他今年二十岁。 – Neutral statement of age.
他今年二十岁了。 – Slightly more emotional/marked: emphasizes the fact that he has now turned 20.