Ik moet eraan denken om mijn telefoon op stil te zetten tijdens de les.

Because denken aan is the fixed combination here.

• denken aan iets = to think about something
• eraan denken often means to remember it / to keep it in mind

In this sentence, eraan refers forward to the action om mijn telefoon op stil te zetten. So:

• Ik moet eraan denken... = I need to remember...

If you said only Ik moet denken, it would just mean I have to think, which is much more general and does not express remembering to do something.

Er + aan = eraan.

This is a very common Dutch structure called a pronominal adverb. Dutch often uses er + preposition instead of repeating a thing or an idea.

Here, aan belongs with denken aan, and er stands in for the thing being thought about. In this sentence, that “thing” is really the whole idea to put my phone on silent during class.

So:

• Ik denk aan mijn telefoon = I am thinking about my phone
• Ik denk eraan = I am thinking about it / I remember it

In Ik moet eraan denken om..., the eraan prepares for the infinitive clause that follows.

Dutch often uses om + te + infinitive to express to do something, especially after verbs like denken when the meaning is remember to or think about doing.

So:

• Ik moet eraan denken om mijn telefoon op stil te zetten. = I need to remember to put my phone on silent.

The structure is:

• moeten
• eraan denken
• om ... te + infinitive

The te is required before the infinitive zetten here.

Usually, learners should keep it: eraan denken om ... te ...

So the safest and most natural version is:

• Ik moet eraan denken om mijn telefoon op stil te zetten.

In some contexts, Dutch speakers may use slightly different patterns, but for a learner, eraan denken om te ... is the standard and most reliable structure.

Moet comes from moeten, which usually means must, have to, or need to.

In this sentence, it often feels a bit softer in English:

• I need to remember to...
• I have to remember to...

It is not necessarily a strict external command. It can also mean a personal reminder:

• I really need to remember this.

So moet can express obligation, necessity, or something you should not forget.

Because Dutch, like English, normally does not use an article together with a possessive before a noun.

So:

• mijn telefoon = my phone
• not de mijn telefoon

The possessive already determines the noun, just like in English.

Other examples:

• zijn jas = his coat
• onze auto = our car

Op stil zetten means to put on silent or to switch to silent mode.

Literally:

• zetten = to put / to set
• op stil = on silent

So the whole expression is the normal Dutch way to say that a phone should stop making sounds.

You may also hear similar expressions such as:

• op trilstand zetten = put on vibrate
• uitzetten = turn off

Here, op stil zetten is the natural choice for put on silent.

In Dutch infinitive constructions, te usually comes directly before the infinitive verb:

• te zetten

Everything else comes before that final verb phrase in a very Dutch-looking order.

So:

• om mijn telefoon op stil te zetten

That is the normal pattern:

• object: mijn telefoon
• complement: op stil
• te + infinitive at the end: te zetten

For English speakers, the important thing is to get used to the verb going to the end of the clause.

No. Tijdens de les can move, because Dutch word order allows a fair amount of flexibility with time expressions.

The given sentence is very natural:

• Ik moet eraan denken om mijn telefoon op stil te zetten tijdens de les.

But you could also say:

• Ik moet er tijdens de les aan denken om mijn telefoon op stil te zetten.
• Tijdens de les moet ik eraan denken om mijn telefoon op stil te zetten.

These versions shift the emphasis slightly, but they are still understandable. The original sentence is straightforward and natural.

It can mean either, depending on context.

In this sentence, tijdens de les usually means:

• during class
• during the lesson

English often prefers during class, while Dutch uses de les very naturally.

So even though les literally looks like lesson, the practical meaning here is often simply class.

No, that would not be correct.

You need the fixed structure:

• eraan denken om ... te ...

not:

• denken aan om ...

The reason is that aan becomes part of eraan, which refers to the following idea. Dutch does not normally say denken aan om te... in this structure.

So use:

• Ik moet eraan denken om...

They are related, but not identical.

• eraan denken = remember to do something / keep it in mind
• onthouden = remember, retain, not forget

In this sentence, eraan denken is the most natural choice because it is about remembering to perform an action in the future.

Compare:

• Ik moet eraan denken om mijn telefoon op stil te zetten. = I need to remember to put my phone on silent.

• Ik moet onthouden dat ik mijn telefoon op stil moet zetten. = I must remember that I need to put my phone on silent.

The second is possible, but less direct and less natural for this situation.

Yes, very often.

Although moeten can literally correspond to must, real-life Dutch often uses it in places where English would say:

• I need to...
• I should...
• I have to...

So the sentence may sound less forceful than the English word must suggests. In everyday usage, it often just means this is something important not to forget.