lǎoshī zài hēibǎn shàng xiě le jǐ gè shēngcí, wǒmen xiān dú, zài xiě jùzi.

Because Chinese usually marks being on the surface of something with 上.

• 在黑板上 = on the blackboard
• 在桌子上 = on the table
• 在墙上 = on the wall

If you say just 在黑板, it sounds incomplete or unnatural in standard Mandarin, because a blackboard is treated as a surface, so 上 is normally needed.

Not exactly. 了 here marks a completed action.

So 老师在黑板上写了几个生词 means the teacher has written / wrote several new words on the blackboard, with emphasis that the writing action is completed.

Important point:

• 了 does not simply equal English past tense.
• It often shows that an action is finished or that a new situation has occurred.

In this sentence, it helps set up the next actions:

• first, the teacher wrote the words
• then, we read and write sentences

So 了 shows the first action is already done before the next steps.

个 is a measure word (also called a classifier).

In Chinese, when you count nouns, you usually need:

• number + measure word + noun

So:

• 几个人 = several people
• 三个苹果 = three apples
• 几个生词 = several new words

Here 个 is the general-purpose measure word. It is often used with many nouns, especially in beginner-level Chinese.

Here it means several / a few, not a question.

Why? Because the whole sentence is a statement, not a question. There is no question mark, and the context is descriptive.

So:

• 几个生词？ could mean How many new words?
• 写了几个生词 in this sentence means wrote several new words

A useful rule:

• In questions, 几 often means how many
• In statements, it can mean a few / several, usually for a small number

They are different words with different meanings, even though the pronunciation is very similar.

• 在 = to be at/in/on; to happen at a place; to indicate location
• 再 = again, then, after that

In this sentence:

• 在黑板上 = on the blackboard
• 再写句子 = then write sentences

So:

• 在 talks about location
• 再 talks about sequence or repetition

This is a very common point of confusion for learners.

先...再... means first..., then...

It is a very common structure for showing order.

In this sentence:

• 我们先读，再写句子。
• We first read, then write sentences.

You can use it in many situations:

• 先吃饭，再学习。 = First eat, then study.
• 先想一想，再回答。 = First think about it, then answer.

So 先 introduces the first action, and 再 introduces the next one.

Because Chinese often omits repeated subjects when they are already clear from context.

The sentence says:

• 我们先读，再写句子。

It could be expanded as:

• 我们先读，我们再写句子。

But repeating 我们 is unnecessary. Once the subject is clear, Chinese often leaves it out in the following clause.

This is very natural and common.

The object is understood from context, so it is omitted.

Earlier in the sentence, we already heard:

• 几个生词 = several new words

So in 我们先读, the natural understood meaning is:

• we first read the new words

Chinese often leaves out objects when they are obvious.

This is similar to English in some situations:

• I already bought the book. Want to read? The word it is understood.

By itself, 读 can mean simply read. It does not automatically tell you whether it is silent or aloud.

However, in a classroom sentence like this, 先读，再写句子, it often suggests read aloud or read through the words first, because that is a common classroom activity.

If Chinese wants to be very specific, it can say:

• 朗读 = read aloud
• 默读 = read silently

So here, 读 is a general read, with the exact style understood from context.

This follows a very common Chinese pattern:

Subject + location phrase + verb + object

So:

• 老师 = subject
• 在黑板上 = location
• 写了 = verb + completion marker
• 几个生词 = object

Chinese often puts location before the main verb:

• 他在家学习。 = He studies at home.
• 我在学校吃饭。 = I eat at school.

So the sentence structure is very normal.

In standard Mandarin, it is best to keep 上 here.

That is because writing happens on the surface of the blackboard, and 上 marks that surface clearly.

So:

• 在黑板上写 = natural
• 在黑板写 = generally not standard

For learners, the safest choice is:

• use 在 + place + 上/里/下 when that postposition is needed

Examples:

• 在书上写字 = write on the book
• 在纸上画画 = draw on paper
• 在教室里学习 = study in the classroom

Because the two parts are doing different things.

• 几个生词 is counting new words, so it needs a number expression and a measure word.
• 写句子 just means write sentences in a general sense. It is not counting them.

If you wanted to count the sentences, then you would use a measure word:

• 写一个句子 = write one sentence
• 写两个句子 = write two sentences

But when speaking generally, Chinese often just uses:

• 写句子
• 说汉语
• 学语法

No measure word is needed unless you are counting.

Yes, 句子 means sentence.

The part 句 can also function as a measure word for sentences or utterances:

• 一句话 = one sentence / one remark
• 两句话 = two sentences

But 句子 itself is a complete noun meaning sentence. So in:

• 写句子 it is simply the object sentences.

Chinese has many words where one part can also appear in other grammatical roles, so this is normal.

Chinese often links related actions with punctuation and context, without needing a direct equivalent of and every time.

Here the comma separates two connected parts:

1. the teacher wrote new words on the board
2. we first read, then write sentences

This feels natural in Chinese. The relationship is clear from context and from the sequence markers 先 and 再.

If needed, Chinese could use extra connecting words, but it often prefers a simpler structure.

Yes, that is close to the structure of the Chinese.

The second part can feel slightly unusual if translated too literally, because Chinese often leaves some things understood:

• the object after 读 is omitted
• the exact object of 写句子 is general
• the tense relationship is not marked the same way English marks it

More natural English usually supplies what is understood:

• The teacher wrote several new words on the blackboard, and we first read them, then write sentences using them.

Chinese does not always need to state everything explicitly if the meaning is obvious from the classroom context.