Jeg bruger indsigelsesblanketten, så længe indsigelsesfristen stadig gælder.

Because Danish normally writes compound nouns as one word, where English often uses two or more words.

So:

• indsigelse = objection / appeal
• blanket = form
• indsigelsesblanket = objection form
• indsigelsesblanketten = the objection form

The same thing happens in:

• indsigelsesfrist = objection deadline
• indsigelsesfristen = the objection deadline

This is very normal in Danish, even when the word looks long to an English speaker.

The ending -en is the definite article attached to the noun. In English, this is usually expressed with the.

So:

• en blanket = a form
• blanketten = the form

And in the compound:

• indsigelsesblanket = objection form
• indsigelsesblanketten = the objection form

Danish often puts the at the end of the noun instead of using a separate word.

For the same reason: -en marks definiteness.

• en frist = a deadline
• fristen = the deadline
• indsigelsesfrist = objection deadline
• indsigelsesfristen = the objection deadline

In this sentence, both nouns are definite because the speaker is referring to specific things:

• the objection form
• the objection deadline

This is very natural in Danish when the context makes it clear which form and which deadline are meant.

Danish often uses the present tense to talk about the future, especially when the meaning is clear from context.

So Jeg bruger ... can mean:

• I use ...
• I am using ...
• I will use ...

depending on context.

Here, English might prefer something like I will use the objection form..., but Danish does not need a separate future form. The present tense is enough.

Here så længe means as long as.

So it introduces a condition:

• så længe indsigelsesfristen stadig gælder = as long as the objection deadline is still valid / still applies

Be careful: så længe can sometimes also be understood as while, depending on context. But in this sentence, as long as is the most natural interpretation because it expresses a condition.

Stadig means still.

It adds the idea that the deadline continues to be valid at the relevant time.

Compare:

• indsigelsesfristen gælder = the objection deadline applies / is valid
• indsigelsesfristen stadig gælder = the objection deadline still applies / is still valid

So stadig emphasizes that the deadline has not expired yet.

Here gælder means something like:

• is valid
• applies
• remains in force

The basic verb at gælde is often used about rules, deadlines, laws, offers, and conditions.

Examples:

• Reglen gælder ikke længere. = The rule no longer applies.
• Tilbuddet gælder til fredag. = The offer is valid until Friday.
• Fristen gælder stadig. = The deadline is still valid / still applies.

So in this sentence, it does not mean matters or concerns; it means is still in effect.

Because after så længe, you have a subordinate clause, and Danish subordinate clauses often place sentence adverbs like stadig before the finite verb.

Main clause word order often looks like this:

• Fristen gælder stadig.

But in a subordinate clause:

• ... så længe fristen stadig gælder.

So the pattern is:

• conjunction: så længe
• subject: indsigelsesfristen
• adverb: stadig
• finite verb: gælder

This is a very important Danish word-order pattern.

Because så længe indsigelsesfristen stadig gælder is a subordinate clause, and in standard Danish writing, subordinate clauses are separated with a comma.

So the structure is:

• main clause: Jeg bruger indsigelsesblanketten
• subordinate clause: så længe indsigelsesfristen stadig gælder

The comma helps show where the subordinate clause begins.

Yes, it could, and that would make the future meaning more explicit.

• Jeg bruger indsigelsesblanketten... = I use / will use the objection form...
• Jeg vil bruge indsigelsesblanketten... = I will use the objection form...

Both are possible, but the version with simple present is very natural in Danish. Danish often relies on context instead of marking the future as directly as English sometimes does.