Das Regal ist staubig, deshalb wische ich es ab.

Because Regal is a neuter noun in German. Its basic article is das.

• das Regal = the shelf
• ein Regal = a shelf

In this sentence, das Regal is the subject of the first clause, so it is in the nominative case.

Because staubig is a predicate adjective here. After verbs like sein, werden, and bleiben, adjectives usually do not take adjective endings.

So:

• Das Regal ist staubig. = correct
• das staubige Regal = also correct, but here the adjective comes before the noun, so it needs an ending

Compare:

• Das Regal ist staubig.
• Ich sehe das staubige Regal.

Deshalb means therefore, that’s why, or for that reason.

It is not a subordinating conjunction like weil. Instead, it is an adverb / connective adverb that links the second clause to the idea in the first clause.

So the sentence structure is basically:

• The shelf is dusty. Therefore, I wipe it off.

Because German main clauses follow the verb-second rule.

That means the finite verb must be in the second position. If deshalb comes first, then the verb must come next:

• Deshalb wische ich es ab.

Structure:

1. deshalb
2. wische
3. ich
4. es
5. ab

This is a very common pattern in German:

• Heute gehe ich nach Hause.
• Dann sehen wir weiter.
• Deshalb wische ich es ab.

Because the sentence contains two main clauses:

1. Das Regal ist staubig
2. deshalb wische ich es ab

In German, when two independent clauses are connected like this, a comma is normally used. The comma helps show the pause and the connection between the two complete thoughts.

Es refers back to das Regal.

Since Regal is a neuter noun, the corresponding pronoun is es. In this sentence, it is the direct object, so it is in the accusative case.

For neuter nouns, nominative and accusative pronouns are both es.

• das Regal → es

So:

• Ich wische das Regal ab.
• Ich wische es ab.

Because the full verb is abwischen, which is a separable verb.

In a normal present-tense main clause, the verb splits:

• Ich wische es ab.

Here:

• wische = the conjugated part
• ab = the separable prefix, placed at the end

Other examples:

• Ich mache die Tür auf. from aufmachen
• Er räumt das Zimmer auf. from aufräumen

But in the infinitive, it stays together:

• Ich will es abwischen.

Wische is the first-person singular present tense form of wischen.

Basic conjugation:

• ich wische
• du wischst
• er/sie/es wischt
• wir wischen
• ihr wischt
• sie/Sie wischen

Since the actual verb here is abwischen, the conjugated form is still based on wischen, and the prefix ab separates:

• ich wische ... ab

Yes, that is also grammatical.

• Deshalb wische ich es ab.
• Ich wische es deshalb ab.

Both mean roughly the same thing, but the emphasis is slightly different.

• Deshalb wische ich es ab. puts more focus on for that reason
• Ich wische es deshalb ab. starts more neutrally with I

German often changes word order to shift emphasis while keeping the same basic meaning.

Yes, but the structure changes.

With deshalb, you have two main clauses:

• Das Regal ist staubig, deshalb wische ich es ab.

With weil, you make the reason into a subordinate clause, so the verb goes to the end of that clause:

• Weil das Regal staubig ist, wische ich es ab.

Both are natural, but they are built differently:

• deshalb = therefore / that’s why
• weil = because

Yes. Common alternatives are:

• deswegen
• darum
• daher

For example:

• Das Regal ist staubig, deswegen wische ich es ab.
• Das Regal ist staubig, darum wische ich es ab.

These are very similar in meaning. For a learner, deshalb, deswegen, and darum are all useful words for that’s why / therefore.