Jeg legger merke til at du er trøtt.

Å legge merke til is a fixed Norwegian expression meaning to notice, to observe, or to take note of.

You should learn it as a whole chunk:

• å legge merke til noe = to notice something
• å legge merke til at ... = to notice that ...

Even though the individual words have other meanings, the expression as a whole is idiomatic. So it is better not to translate it word for word.

Yes. Til belongs to the fixed expression å legge merke til.

A good way to think about it is like an English phrasal verb:

• look at
• find out
• notice does not work this way in English, but legge merke til does in Norwegian.

So in this sentence:

• Jeg legger merke til at du er trøtt

the til is not optional. It is part of the expression.

Because this is an idiomatic expression. In å legge merke til, the word merke appears in a fixed form without an article.

So although et merke can mean a mark / a sign in other contexts, in this expression you do not treat it like a normal noun phrase. You simply memorize:

• legge merke til

not

• legge et merke til

That would sound wrong.

Legger is the present tense of å legge.

A few useful forms are:

• å legge = to lay / put; also used in some idiomatic expressions
• legger = present tense
• la = past tense
• har lagt = past participle

So:

• Jeg legger merke til ... = I notice / I am noticing ...

In Norwegian, the present tense often covers both the English simple present and present progressive.

So:

• Jeg legger merke til ...

can correspond to:

• I notice ...
• I’m noticing ...

In natural English, I notice that you are tired is usually the best translation, but the Norwegian grammar simply uses the ordinary present tense.

At means that and introduces a subordinate clause.

So:

• at du er trøtt = that you are tired

This is very common after verbs of thinking, saying, noticing, knowing, and so on:

• Jeg vet at ... = I know that ...
• Hun sier at ... = She says that ...
• Jeg legger merke til at ... = I notice that ...

As a learner, it is best to keep it.

In informal speech, Norwegian sometimes omits at, just like English sometimes omits that, but at is the safe and standard choice here:

• Jeg legger merke til at du er trøtt.

That is the form learners should use.

Because at introduces a subordinate clause, and subordinate clauses follow their own word-order rules in Norwegian.

In this simple example, the order looks very normal:

• du er trøtt = you are tired

But the rule becomes clearer if you add ikke:

• Jeg legger merke til at du ikke er trøtt. = I notice that you are not tired.

In subordinate clauses, words like ikke come before the finite verb more often than they do in main clauses. So this is an important structure to get used to.

Trøtt usually means tired or sleepy.

It can refer to needing sleep, lacking energy, or looking tired.

Sliten overlaps with it, but often means more like:

• worn out
• exhausted
• drained
• tired after effort

So:

• trøtt = sleepy / tired
• sliten = tired out / worn out

In many situations both are possible, but they are not always identical.

Yes, absolutely. That is very natural.

• Jeg merker at du er trøtt = I notice that you are tired
• Jeg legger merke til at du er trøtt = I notice that you are tired

The difference is subtle:

• merker can feel a bit more direct and everyday
• legger merke til can feel slightly more deliberate or observational

Both are correct and useful.

It is natural, but the tone depends on the situation and your intonation.

It can sound:

• caring: if you are concerned about someone
• neutral: if you are simply observing
• slightly pointed: if you are commenting on how they look or behave

If you want something softer, you could say:

• Du virker trøtt. = You seem tired.
• Er du trøtt? = Are you tired?

So the sentence itself is fine, but context matters.

This is Bokmål.

A Nynorsk version would typically be:

• Eg legg merke til at du er trøytt.

Some differences are:

• Jeg → Eg
• legger → legg
• trøtt → trøytt

So if you are studying standard Norwegian as most learners do, the original sentence is Bokmål.

The difficult part for many English speakers is ø.

In trøtt, the ø is a rounded vowel that does not exist in standard English. You make a sound somewhat like the vowel in bird or burn, but with rounded lips.

A rough guide:

• trøtt sounds approximately like trutt, but with rounded lips and a different vowel quality

Also note:

• the final tt is pronounced clearly
• the word is short and crisp

If you are learning pronunciation, ø is worth practicing separately because it appears in many common Norwegian words.