Hvis jeg spilder kaffe på bordet, tørrer jeg det op med en ren klud.

Because Danish main clauses follow the verb-second rule.

The first part, Hvis jeg spilder kaffe på bordet, is a subordinate clause starting with hvis.

When that whole clause comes first, the main clause still has to put the finite verb in second position, so you get:

• Hvis jeg spilder kaffe på bordet, tørrer jeg det op
• not Hvis jeg spilder kaffe på bordet, jeg tørrer det op

So the pattern is:

• [subordinate clause], [verb] [subject] ...

This is very common in Danish:

• Når jeg kommer hjem, laver jeg mad
• Hvis det regner, bliver vi inde

Tørre op means to wipe up or clean up a spill.

In Danish, some verbs combine with a small particle like op, af, ud, etc. These often work a bit like English phrasal verbs.

So here:

• tørre op = wipe up
• tørre det op = wipe it up

The op is separated because Danish often splits these verb-particle combinations in normal sentences.

Compare:

• Jeg tørrer det op = I wipe it up
• Jeg vil tørre det op = I want to wipe it up

This is very similar to English wipe it up.

That is a very natural question.

Strictly speaking, kaffe is a common-gender noun, so if you were referring directly to the coffee as a noun, you might expect den.

But in this sentence, det is very natural because it refers less to the noun kaffe itself and more to the spilled stuff / the mess / what was spilled. Danish often uses det in that kind of situation.

So:

• spilder kaffe = spill coffee
• tørrer det op = wipe it up

This is idiomatic Danish. An English speaker should not try to match det too mechanically to the grammatical gender of the earlier noun here.

Because Danish often uses the present tense for general situations, habits, and even future-like conditions.

So:

• Hvis jeg spilder kaffe på bordet, tørrer jeg det op

can mean something like:

• If I spill coffee on the table, I wipe it up
• or more naturally in English, If I spill coffee on the table, I’ll wipe it up

Danish does not need a special future form here.

If you wanted a past meaning, you would change the tense:

• Hvis jeg spildte kaffe på bordet, tørrede jeg det op
  = If I spilled coffee on the table, I wiped it up

In this sentence, på bordet means on the table, but in English we might often translate the whole idea as spill coffee onto the table.

Danish uses på in many cases where English might choose either on or onto, depending on the situation.

So:

• kaffe på bordet = coffee on the table
• spilde kaffe på bordet = spill coffee on/onto the table

The Danish wording is completely normal.

Because Danish usually adds the definite article to the end of the noun.

So:

• bord = table
• bordet = the table

This is one of the big differences from English.

A few examples:

• en klud = a cloth
• kluden = the cloth
• et bord = a table
• bordet = the table

So på bordet literally looks like on table-the, but in normal English that is simply on the table.

Because klud is a common-gender noun, an en-word.

That affects both the article and the adjective form:

• en klud = a cloth
• en ren klud = a clean cloth

In the indefinite singular common gender, the adjective usually has no extra ending.

Compare:

• en ren klud = a clean cloth
• et rent håndklæde = a clean towel
• den rene klud = the clean cloth

So the pattern is:

• en + adjective + common-gender noun
• et + adjective + -t + neuter noun
• den/det + adjective + -e + definite noun

Use hvis for a real condition:

• Hvis jeg spilder kaffe, tørrer jeg det op
  = If I spill coffee, I wipe it up

Use om when English means whether/if in an indirect question:

• Jeg ved ikke, om jeg kommer
  = I don’t know if/whether I’m coming

So here, hvis is correct because the sentence is about a condition: if this happens, then I do that.

Sometimes you can leave it out if the context is already very clear, but here tørrer jeg det op is the most natural and complete form.

Compare:

• Jeg tørrer det op = I wipe it up
• Jeg tørrer op = I’m wiping up / I’m doing the wiping up

The version without det sounds less specifically tied to the spilled coffee. With det, the sentence clearly points to the spill.

They are related, but they focus on different things.

• tørre op = wipe up a spill
• tørre af = wipe off / wipe something clean, often a surface

So:

• Jeg tørrer det op
  = I wipe it up
  focus: the spilled liquid or mess

• Jeg tørrer bordet af
  = I wipe the table off
  focus: the table as a surface

In your sentence, the speaker is cleaning up the spilled coffee, so tørre op is the right choice.

Spilde is the normal verb for spill in this context.

So:

• spilde kaffe = spill coffee
• spilde vand = spill water
• spilde mælk = spill milk

That is the most natural choice for accidentally letting liquid fall where it should not.

A learner might think of other verbs, but spilde is the standard one for this meaning.