Tijdens de discussie bleef Anna rustig, ook toen Tom en Sofie het oneens waren.

Tijdens means during.

It is a preposition, so it is followed by a noun phrase:

• tijdens de discussie = during the discussion
• tijdens de les = during the lesson
• tijdens het eten = during the meal / while eating

In this sentence, Tijdens de discussie sets the time frame for the whole statement.

This is because Dutch main clauses usually follow the V2 rule: the finite verb must be in the second position.

So if you start the sentence with something other than the subject, the verb still has to come second:

• Anna bleef rustig.
• Tijdens de discussie bleef Anna rustig.

Here, Tijdens de discussie takes the first position, so bleef must come next, and the subject Anna comes after that.

This is a very common Dutch word order pattern.

Bleef is the past tense of blijven, which means to stay or to remain.

So:

• Anna was rustig = Anna was calm
• Anna bleef rustig = Anna stayed/remained calm

Using bleef emphasizes that she continued to be calm, especially in a situation where that might have changed.

That fits this sentence well, because the discussion involved disagreement, but Anna still remained calm.

Because rustig is being used as a predicate adjective, after the verb bleef.

In Dutch, predicate adjectives do not take the -e ending:

• Anna bleef rustig.
• Hij is moe.
• De kinderen waren stil.

You usually add -e when the adjective comes before a noun:

• de rustige Anna
• een rustige stem
• de stille kinderen

So here rustig stays plain because it comes after the verb.

Literally, ook toen means also when or also then when, but in natural English it often comes across as even when or including when.

In this sentence, it means that Anna stayed calm in that situation too:

• ook toen Tom en Sofie het oneens waren

So the idea is:

• she was calm during the discussion,
• and that was still true when Tom and Sofie were disagreeing.

It adds emphasis: that was a moment when things could have become tense, but Anna still stayed calm.

Here, toen means when.

Dutch toen can have different uses:

• as an adverb: then
• as a conjunction: when (especially in past-time contexts)

In this sentence, it introduces a clause:

• toen Tom en Sofie het oneens waren

So here it is a conjunction, meaning when.

Because this is a subordinate clause introduced by toen.

In Dutch subordinate clauses, the conjugated verb usually goes to the end:

• toen Tom en Sofie het oneens waren
• omdat hij moe was
• dat ze naar huis gingen

So:

• main clause: Anna bleef rustig
• subordinate clause: toen Tom en Sofie het oneens waren

That final waren is standard Dutch subordinate-clause word order.

Het oneens zijn means to disagree or to be in disagreement.

It is a fixed expression. Related expressions are:

• het eens zijn = to agree
• het oneens zijn = to disagree
• het met iemand eens zijn = to agree with someone
• het met iemand oneens zijn = to disagree with someone

So in the sentence:

• Tom en Sofie waren het oneens
  = Tom and Sofie disagreed / were in disagreement

The sentence does not say exactly what they disagreed about, because that is already understood from the context.

The het is part of the fixed idiomatic expression het eens zijn / het oneens zijn.

It does not translate neatly word-for-word into English. English just says:

• they agreed
• they disagreed

But Dutch says:

• zij waren het eens
• zij waren het oneens

So the best way to learn this is as a chunk:

• het eens zijn
• het oneens zijn

You do not need to assign a strong separate meaning to het here; it is simply part of the expression.

Because the subject is Tom en Sofie, which is plural.

So Dutch uses the plural past tense form of zijn:

• ik was
• hij/zij was
• wij/jullie/zij waren

Since Tom en Sofie = they, the correct form is waren.

Yes. Het niet eens zijn also means to disagree.

So these are both possible:

• Tom en Sofie waren het oneens
• Tom en Sofie waren het niet eens

Both are natural. Oneens is a bit more compact and explicit as a single adjective. Niet eens is also very common.

One thing to watch out for: niet eens can also mean not even in other contexts, so learners sometimes find oneens clearer.

Yes, oneens is written as one word.

It is an adjective meaning in disagreement.

Examples:

• Wij zijn het oneens.
• Ze bleven het oneens.

This is different from separate combinations like niet eens, which are written as two words.