Learn Hardcore Danish: Jeg skulle læse bogen i aften, men jeg er for træt. - I was supposed to read the book tonight, but I am too tired.

Here Jeg skulle læse bogen i aften is best understood as I was supposed to read the book tonight or I was going to read the book tonight. It expresses a plan, arrangement, or obligation that existed, but which (because of the second half of the sentence) is now not going to happen.

If you use skal instead (Jeg skal læse bogen i aften), it normally means I’m going to / I have to read the book tonight – a plan or obligation that still stands.

Why is skulle in the past tense if the time expression is i aften (tonight)?

Danish (like English) can use a past tense modal to talk about a plan or expectation that is no longer valid.

Compare English:

I was going to read the book tonight, but I’m too tired.

The was going to is past, but the time (tonight) is still in the future from when the plan was made. The past tense shows distance from reality: the plan existed, but reality has changed.

Jeg skulle læse bogen i aften, men jeg er for træt works the same way: the plan was to read it tonight, but now that isn’t going to happen.

Could I say Jeg skal læse bogen i aften, men jeg er for træt instead? What would be the difference?

You can say it, but the meaning changes.

Jeg skulle læse bogen i aften, men jeg er for træt.
⇒ The plan was to read it tonight, but because I’m too tired, I probably won’t do it.
Jeg skal læse bogen i aften, men jeg er for træt.
⇒ I’m supposed to / have to read it tonight, but I’m very tired.
This often sounds more like a complaint about an obligation that still exists: I still have to do it, even though I’m tired.

So skulle suggests a canceled or doubtful plan; skal suggests an obligation or arrangement that still holds.

Why is there no at before læse?

In Danish, modal verbs are followed directly by the infinitive, without at:

Jeg skal læse bogen.
Jeg kan læse bogen.
Jeg vil læse bogen.
Jeg må læse bogen.
Jeg skulle læse bogen.

You only use at with non-modal verbs before an infinitive, e.g. Jeg begynder at læse bogen (I’m starting to read the book).

Why is it bogen and not en bog or den bog?

Danish usually marks definiteness with a suffix on the noun:

en bog = a book (indefinite)
bogen = the book (definite)

So bogen means the book.

den bog is also possible, but it’s used differently:

With an adjective: den gode bog (the good book)
For contrast / emphasis: ikke den bog, men den anden (not that book, but the other one)

Without an adjective and without contrast, bogen is the normal, neutral way to say the book.

Where can I put i aften in the sentence? Is I aften skulle jeg læse bogen also correct?

Yes, I aften skulle jeg læse bogen is correct. Danish just has to keep the finite verb in second position in main clauses (the V2 rule). Some common options:

Jeg skulle læse bogen i aften. (neutral word order)
Jeg skulle i aften læse bogen. (slight emphasis on tonight)
I aften skulle jeg læse bogen. (stronger emphasis on tonight; you’re contrasting tonight with some other time)

All three are grammatically correct. The version you gave (Jeg skulle læse bogen i aften) is the most neutral, everyday option.

What does for træt mean exactly, and how is it different from meget træt?

for træt = too tired (so tired that you cannot or will not do the action)
⇒ It usually implies that the tiredness prevents you from doing something.
meget træt = very tired
⇒ Just describes a high degree of tiredness; it doesn’t automatically say whether you can still do the thing or not.

So:

Jeg er for træt (til at læse bogen). = I’m too tired (to read the book).
Jeg er meget træt, men jeg læser bogen alligevel. = I’m very tired, but I’ll read the book anyway.

Could I say … men jeg var for træt instead of … men jeg er for træt? Would that change the meaning?

Yes, you can, and it does change the time frame:

… men jeg er for træt.
⇒ I’m too tired now, at the moment of speaking.
… men jeg var for træt.
⇒ I was too tired at that time in the past.
This would usually be part of a story about an earlier situation, not about right now.

In the original sentence, er makes sense because you’re explaining why the plan for tonight is being canceled now.

Is the comma before men necessary in Danish?

Yes, in standard written Danish you normally put a comma before men when it connects two main clauses:

Jeg skulle læse bogen i aften, men jeg er for træt.

This is similar to English: I was going to read the book tonight, but I’m too tired.

Leaving out the comma here would generally be seen as incorrect or very careless writing.

Does i aften literally mean in the evening? Can it also mean tonight, and when do you use i nat?

Literally, i aften is in the evening, but in practice it often corresponds to English tonight when you’re talking about what you’ll do this evening (typically the time between late afternoon and when you go to bed).

i aften = this evening / tonight (evening part)
i nat = tonight (during the night, when people are normally asleep)

So:

Jeg læser bogen i aften. = I’ll read the book this evening / tonight.
Jeg arbejder i nat. = I’m working tonight (night shift, during the night).

How do you pronounce jeg, skulle, læse, bogen, and træt?

Approximate pronunciations (IPA + rough English hint):

jeg – /jɑj/
Similar to “yigh” in English (like guy with a y), but shorter and more relaxed. In fast speech it can sound like a very quick “yai” or “ya”.
skulle – /ˈsguˀlə/
Start with sk that sounds a bit like “sg”, then a short oo as in good, and a very light -leh: SGOO-leh, with a little catch in the voice after sgoo.
læse – /ˈlɛːsə/
The æ is like a long e in bed, not like say. Something like LEH-seh, with the first vowel held a bit longer.
bogen – /ˈbɔwən/
The bo is like a short boh with an ow as in low, then a weak -en: roughly BOH-ən. The g is not pronounced as a hard g.
træt – /tʁaˀd/
The r is a throaty sound, and the vowel is like a in father, but shorter. Final t is not a clear [t]; it’s more like a soft d with a glottal stop: roughly TRA(d) with the end cut off sharply.

These are approximations; listening to native audio will help you fine‑tune the sounds.