Vores mor lader os altid ordne loftet selv, men hun hjælper, hvis noget er for tungt.

In Danish, a possessive like vores already makes the noun definite, so you do not add a separate article.
So vores mor simply means our mother.

Also, modern Danish normally uses vores for our. You may sometimes see the older/literary form vor, but vores is the everyday choice.

Because os is the object form of we. After lader, the people being allowed to do something are the object:

vores mor lader os ordne loftet
= our mother lets us sort out the attic

So the pattern is:

lade + object + infinitive

That is why os is correct, not vi.

After lade, Danish uses a bare infinitive, just like English often does after let.

So:

• hun lader os ordne loftet
• not hun lader os at ordne loftet

This is a standard construction: lade nogen gøre noget = let someone do something.

Here lade means let or allow. It does not mean that the mother is doing the action herself; it means she allows us to do it.

So lader os ordne loftet selv means she lets us handle the attic ourselves.

Ordne is a very flexible verb. Depending on context, it can mean organize, sort out, fix, arrange, or tidy up.

With loftet, it most naturally suggests something like sort out or tidy up the attic, rather than just performing one specific physical action.

Yes. Loft can mean either ceiling or attic, depending on context.

The ending -et is the definite ending, so loftet means the attic or the ceiling. In this sentence, the context strongly points to the attic, because ordne loftet sounds like sorting out or tidying up a space, not dealing with a ceiling.

Selv adds the idea of ourselves or on our own. It emphasizes that they do the job without the mother doing it for them.

So:

• lader os ordne loftet = lets us sort out the attic
• lader os ordne loftet selv = lets us sort out the attic ourselves

That word order is natural in Danish. After the finite verb lader, the unstressed object pronoun os comes early, and adverbs like altid often come before the infinitive phrase.

So lader os altid ordne loftet selv is the normal pattern.
A learner may expect something closer to English word order, but Danish places short object pronouns quite early.

Because the object is understood from the context. It clearly means she helps us, so Danish can leave it unstated.

You could say men hun hjælper os, but it is not necessary here. English does the same thing sometimes: but she helps if something is too heavy.

Danish normally uses commas between clauses more consistently than English does.

• ..., men hun hjælper ...: comma before the coordinating clause with men
• ..., hvis noget er for tungt: comma before the subordinate clause introduced by hvis

So the punctuation is very standard Danish clause punctuation.

There are three useful points here:

1. noget means something here.
2. for before an adjective means too in the sense of excessively.
3. tungt has -t because noget is grammatically neuter.

So:

• tung = heavy
• for tungt = too heavy
• noget er for tungt = something is too heavy

Because hvis introduces a subordinate clause, and subordinate clauses do not use main-clause V2 word order.

So the normal order is:

hvis + subject + verb

That gives:

hvis noget er for tungt

A main clause in Danish often has the finite verb in second position, but this clause is not a main clause.