Ich überlege, ob ich heute Müsli oder Quark zum Frühstück nehme.

Ob introduces an indirect yes/no question.

So:

• Ich überlege. = I’m thinking / considering.
• ob ich heute Müsli oder Quark zum Frühstück nehme = whether I’ll have muesli or quark for breakfast today

A native English speaker can often translate ob as whether or sometimes if.

In this sentence, the speaker is not asking a direct question. They are reporting a thought: I’m considering whether...

Because the part after ob is a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Compare:

• Main clause: Ich nehme heute Müsli.
• Subordinate clause: ..., ob ich heute Müsli nehme.

So in:

• Ich überlege, ob ich heute Müsli oder Quark zum Frühstück nehme.

the verb nehme moves to the end of the ob-clause.

This is one of the most important German word-order rules.

In German, subordinate clauses are normally separated from the main clause with a comma.

So:

• Ich überlege, ob ...

needs a comma because ob ich heute Müsli oder Quark zum Frühstück nehme is a subordinate clause.

German uses commas more consistently than English in structures like this.

Both can work, but nehmen is very common when talking about choosing or having food.

Here, nehmen means something like:

• to have
• to take
• to choose

So:

• Was nimmst du zum Frühstück? = What are you having for breakfast?

If you said esse, that would focus more directly on the act of eating:

• ..., ob ich heute Müsli oder Quark zum Frühstück esse.

That is also grammatically correct, but nehme sounds very natural when someone is deciding between options.

Ich überlege means I’m thinking about it, I’m considering it, or I’m trying to decide.

It does not mean just having a random thought. It suggests some deliberate consideration.

In this sentence:

• Ich überlege, ob ... = I’m thinking about whether ...
• I’m trying to decide whether ...

So the whole sentence has the sense of weighing two breakfast options.

Zum is a contraction of zu dem:

• zu dem → zum

In this expression, zum Frühstück means:

• for breakfast
• literally something like at breakfast / for the breakfast meal

This is a very common German way to talk about food connected with a meal:

• zum Frühstück
• zum Mittagessen
• zum Abendessen

So Müsli oder Quark zum Frühstück means muesli or quark for breakfast.

It is dative.

That is because zu takes the dative case:

• zu + dem Frühstück → zum Frühstück

So:

• dem Frühstück is dative singular
• zum is just the shortened form

Even if you translate it as for breakfast, the German structure is built with zu + dative.

In German, foods are often used without an article when talking about them in a general, non-specific way.

So:

• Ich esse Müsli.
• Ich nehme Quark.

means I eat/have muesli or quark, not a specific previously mentioned one.

You could add an article in some contexts, but then the meaning becomes more specific:

• das Müsli = the muesli
• den Quark = the quark

In this sentence, the speaker is just choosing between two kinds of food in general, so no article is needed.

German word order is flexible, but not random.

Inside the ob-clause:

• ich = subject
• heute = time expression
• Müsli oder Quark = object
• zum Frühstück = prepositional phrase
• nehme = verb at the end

This order is very natural. A rough pattern is:

• subject
• time
• other information
• verb at the end

You could move some parts for emphasis, but the sentence as given sounds neutral and idiomatic.

For example, these are also possible:

• ..., ob ich zum Frühstück heute Müsli oder Quark nehme.
• ..., ob ich heute zum Frühstück Müsli oder Quark nehme.

But the original version is very standard.

Because ob means whether, while wenn usually means if/when in a conditional or time sense.

Here the speaker is deciding between two possibilities, so it is an indirect yes/no question:

• Ich überlege, ob ich ... nehme. = I’m wondering whether I’ll have ...

You would not normally use wenn here.

Compare:

• Ich weiß nicht, ob er kommt. = I don’t know whether he’s coming.
• Wenn er kommt, essen wir. = If/When he comes, we’ll eat.

So ob is the correct choice for whether.

Because Müsli is a noun, and all nouns are capitalized in German.

So in this sentence, these are capitalized because they are nouns:

• Ich
• Müsli
• Quark
• Frühstück

This is a basic but very important German spelling rule.

Yes, it is spelled the same, but here it means the food: a fresh dairy product somewhat like a thick soft cheese or strained yogurt.

In everyday German:

• Quark usually means the food

The physics meaning exists too, but in a sentence about breakfast, only the food meaning makes sense.

So a learner should understand Müsli oder Quark as two breakfast foods, not anything scientific.

Yes. That would also be natural.

Compare:

• Ich überlege, ob ich heute Müsli oder Quark zum Frühstück nehme.
• Ich denke darüber nach, ob ich heute Müsli oder Quark zum Frühstück nehme.

Both mean that you are considering something.
But überlegen is shorter and very common for making a decision.

So the original sentence is perfectly natural and probably the most straightforward everyday version.