Jeg behøver ikke at skrive hele opgaven i dag, så længe jeg bliver færdig før fredag.

In a normal main clause, Danish places the sentence adverbial ikke after the finite verb:

• Jeg behøver ikke ...
• Han kommer ikke i dag.

So behøver is the conjugated verb, and ikke follows it. This is one of the most important word-order patterns in Danish main clauses.

If you moved ikke before the verb, it would sound wrong in a standard sentence like this.

Behøver means need to or have to, usually in the sense of necessity.

• Jeg behøver ikke at skrive ... = I don’t need to write ...

This is different from skal, which often means must, have to, or am supposed to:

• Jeg skal skrive opgaven i dag = I have to / am supposed to write the assignment today
• Jeg behøver ikke at skrive opgaven i dag = I don’t need to write the assignment today

So behøver ikke is about a lack of necessity, not prohibition.

After behøver, Danish often uses at + infinitive:

• behøver at skrive
• behøver at vente
• behøver at gå

So:

• Jeg behøver ikke at skrive ...

In everyday Danish, some speakers may omit at in certain contexts, but behøver ikke at skrive is very standard and safe to learn.

Yes, many Danes do say that, especially in speech:

• Jeg behøver ikke skrive hele opgaven i dag

This is common and natural. However, Jeg behøver ikke at skrive ... is also correct and often feels a little more careful or complete in writing. As a learner, you can use either, but keeping at is a good default.

Hele means the whole or all of.

• hele opgaven = the whole assignment
• hele dagen = the whole day
• hele bogen = the whole book

So the sentence does not mean that you do not need to write anything today. It means you do not need to write all of the assignment today.

That suggests you may still write part of it today.

In Danish, when hel means whole/entire and modifies a noun with a definite ending, it usually comes before the noun like this:

• hele opgaven
• hele bogen
• hele dagen

This is the normal pattern.

Den hele opgave is not the standard form here. Learners often expect an extra article because of English structure, but Danish handles definiteness differently.

Here så længe means as long as in the sense of provided that or on condition that.

• ..., så længe jeg bliver færdig før fredag.
• ..., as long as I finish before Friday.

Be careful: så længe can also literally mean for that long / for so long in other contexts. But in this sentence, it introduces a condition.

Because after så længe, you get a subordinate clause, and Danish subordinate clauses usually keep the subject before the verb:

• så længe jeg bliver færdig før fredag

Compare that with main clause word order:

• Jeg bliver færdig før fredag

In subordinate clauses, sentence adverbials like ikke usually come before the finite verb:

• ... fordi jeg ikke kommer
• ... hvis jeg ikke har tid

That is a major contrast with main clauses, where ikke comes after the finite verb.

Bliver færdig means get finished, finish, or be done.

• jeg bliver færdig før fredag = I finish / get done before Friday

This is a very common and natural Danish way to talk about completing something.

Other verbs are possible, but they are not always as natural here:

• færdiggøre = complete, but it can sound more formal
• afslutte = finish/conclude, also possible, but often less everyday than blive færdig

So bliver færdig is probably the most natural everyday choice.

Both can be possible, but they are not always exactly identical.

• før fredag = before Friday
• inden fredag = by Friday / before Friday

In this sentence, før fredag works well because it focuses on a time earlier than Friday.

If you said:

• ... så længe jeg bliver færdig inden fredag

that would also be very natural, and many speakers might even prefer it in everyday speech because it strongly suggests no later than Friday.

So both are possible here, but inden fredag can feel slightly closer to English by Friday.

By itself, i dag simply means today.

In this sentence:

• Jeg behøver ikke at skrive hele opgaven i dag

it means that completing the whole assignment is not necessary today. It does not specifically mean later today unless the context suggests that.

Not necessarily, but it suggests that completing all of it today is unnecessary.

The exact implication is:

• finishing the entire assignment today is not required
• the important condition is finishing it before Friday

It leaves open several possibilities:

• you write part of it today and finish later
• you do none of it today and finish on another day
• you finish all of it today anyway

The only stated condition is being done before Friday.

No. Behøver ikke means do not need to.

• Jeg behøver ikke at gå = I don’t need to go

If you want must not or am not allowed to, Danish uses other expressions, such as:

• Jeg må ikke gå = I must not / I’m not allowed to go

This is an important difference:

• behøver ikke = no necessity
• må ikke = prohibition

A very natural spoken version could be:

• Jeg behøver ikke skrive hele opgaven i dag, så længe jeg bliver færdig inden fredag.

What changed?

• at is omitted, which is common in speech
• inden fredag is a very natural alternative to før fredag

But the original sentence is already correct and natural. The spoken version is just a bit more conversational.