De professor zei dat ik niet haastig moest antwoorden, maar eerst rustig moest nadenken over mijn samenvatting.

Because dat introduces a subordinate clause, and in Dutch subordinate clauses the verb(s) usually move to the end.

So:

• main clause: De professor zei
• subordinate clause: dat ik niet haastig moest antwoorden, maar eerst rustig moest nadenken over mijn samenvatting

Inside that subordinate clause, the subject ik comes early, and the verbal part goes later:

• moest antwoorden
• moest nadenken

That is why Dutch does not follow the English order that I should not answer hastily. Dutch prefers the verb cluster near the end of the clause.

Here moest is the past tense of moeten, but in context it is best understood as reported advice, instruction, or obligation.

So ik moest niet haastig antwoorden can mean things like:

• I was not supposed to answer hastily
• I should not answer hastily
• I had to avoid answering hastily

Because the reporting verb is in the past — zei — Dutch often uses the past form moest in the reported clause.

So this is not only about past time; it also reflects the professor’s instruction as reported speech.

Because there are two coordinated actions:

• niet haastig moest antwoorden
• maar eerst rustig moest nadenken over mijn samenvatting

Both belong to the same subordinate clause, but each part has its own verbal structure. Repeating moest makes that very clear and sounds natural.

You can think of it as:

• The professor said that I should not answer hastily,
• but that I should first think calmly about my summary.

So the repetition helps show that both actions are separately governed by moest.

Because they come after the modal verb moest.

In Dutch, modal verbs such as:

• moeten
• kunnen
• mogen
• willen
• zullen

are normally followed by an infinitive, not by a conjugated verb.

So:

• ik moest antwoorden
• ik moest nadenken

not:

• ik moest antwoordde
• ik moest dacht

This is similar to English:

• I had to answer
• I had to think

After a modal verb like moeten, Dutch uses the bare infinitive, without te.

So you say:

• ik moest antwoorden
• ik moest nadenken

not:

• ik moest te antwoorden
• ik moest te nadenken

This is an important rule. Many Dutch infinitives do use te in other structures, but not after modal verbs.

Because niet usually comes before the part it negates.

Here the speaker is negating the manner of answering:

• not in a hasty way

So niet comes before haastig:

• niet haastig antwoorden

That means the instruction is not necessarily do not answer at all, but rather do not answer hastily.

Word order helps show what is being negated.

Not quite.

• haastig means hasty, hurried, rushed
• snel means fast, quick

So haastig antwoorden suggests answering in a rushed, insufficiently thoughtful way.

That fits well with the second half of the sentence, where the professor says to think calmly first. So haastig is more appropriate than snel here, because the idea is not just speed, but careless haste.

Rustig describes the manner of thinking: calmly.

In this clause, Dutch places the adverb before the verb cluster it modifies:

• eerst rustig moest nadenken

That gives the sense:

• first had to think calmly

Dutch adverb placement is flexible in some cases, but this position is very natural. It keeps rustig close to the verbal idea it modifies.

Because nadenken commonly goes with the preposition over when you mean think about something.

So:

• nadenken over een probleem
• nadenken over mijn samenvatting

This is simply the standard combination. English learners often want to translate word for word, but prepositions often differ between languages and must be learned together with the verb.

A good habit is to learn it as one unit:

• nadenken over = to think about

It is the object of the prepositional phrase over mijn samenvatting, which belongs with nadenken.

So the structure is:

• nadenken over mijn samenvatting

meaning:

• to think about my summary

The noun samenvatting means summary, and mijn means my.

So the professor said the speaker should first think calmly about their summary.

Because maar introduces a contrasting second part:

• not answer hastily,
• but first think calmly about my summary

The comma helps separate these two contrasted ideas. This is very normal in Dutch punctuation, especially when maar connects two longer clause parts.

Not in the same way.

• antwoorden = to answer, usually without a direct object, or answering in general
• beantwoorden = to answer something, usually a direct object such as a question

Examples:

• Ik antwoordde snel.
• Ik beantwoordde de vraag.

In your sentence, the focus is on the manner of answering, not on answering a specific object, so antwoorden fits naturally.