Så lenge avtalevilkårene er tydelige, kan vi velge tilbudet.

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

So:

• Så lenge avtalevilkårene er tydelige = As long as the contract terms are clear

This is a very common expression in Norwegian.

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

Not exactly.

• hvis = if
• så lenge = as long as / provided that

Both can introduce a condition, but så lenge often suggests that something is true only under that condition.

Compare:

• Hvis avtalevilkårene er tydelige, kan vi velge tilbudet.
  = If the contract terms are clear, we can choose the offer.

• Så lenge avtalevilkårene er tydelige, kan vi velge tilbudet.
  = As long as the contract terms are clear, we can choose the offer.

The second one sounds a bit more like that condition is enough.

Norwegian very often forms compound nouns as one word.

avtalevilkårene can be broken down like this:

• avtale = agreement / contract
• vilkår = terms / conditions
• -ene = the definite plural ending

So:

• avtalevilkår = contract terms
• avtalevilkårene = the contract terms

This is completely normal in Norwegian. Where English often uses several words, Norwegian often joins them into one compound.

Because it is in the definite plural.

The base noun is:

• et vilkår = a term / condition

Plural forms:

• vilkår = terms / conditions
• vilkårene = the terms / the conditions

In the compound:

• avtalevilkår = contract terms
• avtalevilkårene = the contract terms

So -ne is like saying the for a plural noun.

Because the adjective agrees with a plural definite noun.

Here the noun is:

• avtalevilkårene = the contract terms

That is plural, so the adjective takes the plural/definite form:

• tydelig = clear
• tydelige = clear (plural / definite / some other agreeing contexts)

So:

• Vilkåret er tydelig = The term is clear
• Vilkårene er tydelige = The terms are clear

Because the sentence begins with a subordinate clause:

• Så lenge avtalevilkårene er tydelige = subordinate clause
• kan vi velge tilbudet = main clause

In standard written Norwegian, it is normal to use a comma between them:

• Så lenge ..., kan vi ...

This is similar to English when a dependent clause comes first.

This is because Norwegian follows the V2 rule in main clauses: the finite verb usually comes in the second position.

The sentence starts with the subordinate clause:

• Så lenge avtalevilkårene er tydelige

That whole clause counts as the first element. So in the main clause, the verb must come next:

• kan = second position
• vi = comes after the verb

So:

• Så lenge avtalevilkårene er tydelige, kan vi velge tilbudet.

If there were no fronted clause, you would normally say:

• Vi kan velge tilbudet.

kan is the present tense of kunne, a modal verb.

Here it means can / are able to / may depending on context. In this sentence, it most naturally means something like:

• we can choose the offer
• we are able to choose the offer
• we may choose the offer

Because it is a modal verb, it is followed by the infinitive without å:

• kan velge
• not kan å velge

This is just like English can choose, not can to choose.

Because after modal verbs like kan, Norwegian uses the infinitive without å.

Common modal verbs include:

• kan = can
• skal = shall / will / is going to
• må = must
• vil = want to / will
• bør = should

So you get:

• kan velge
• må gå
• vil spise

not:

• kan å velge
• må å gå
• vil å spise

Tilbudet is the definite singular form:

• et tilbud = an offer
• tilbudet = the offer

So the sentence refers to a specific offer, not just any offer.

Compare:

• Vi kan velge et tilbud. = We can choose an offer.
• Vi kan velge tilbudet. = We can choose the offer.

Tilbud can mean several things depending on context, such as:

• offer
• proposal
• deal
• special offer / discount offer

In this sentence, because of avtalevilkårene (the contract terms), tilbudet most likely means something like:

• the offer
• the proposal

rather than a supermarket-style special deal, although the exact meaning depends on the wider context.

Yes. Both are possible, depending on context.

• avtale can mean agreement or contract
• vilkår means terms / conditions

So avtalevilkårene could be:

• the agreement terms
• the contract terms
• the terms of the agreement/contract

In natural English, contract terms is often the smoothest translation.

Yes, it is completely natural.

It follows a very common pattern:

• subordinate clause first
• comma
• main clause with V2 word order

Pattern:

• Så lenge + clause, verb + subject + rest

Example:

• Så lenge været er fint, kan vi gå ut.
• As long as the weather is nice, we can go out.

So your sentence is a very standard and useful model to learn from.