Meine Schwester will langsam bohren, und ich halte die Wasserwaage an die Wand.

Because will is the 3rd person singular present form of the German verb wollen.

• ich will = I want to
• du willst = you want to
• sie will = she wants to

So Meine Schwester will langsam bohren means My sister wants to drill slowly or intends to drill slowly.

This is a very common false friend for English speakers, because German will usually does not mean the future tense. German often uses the plain present tense for future meaning, or sometimes werden.

Because wollen is a modal verb, and modal verbs usually send the main verb to the end in the infinitive.

So the pattern is:

• subject + modal verb + ... + infinitive

Here:

• Meine Schwester = subject
• will = modal verb
• langsam = adverb
• bohren = infinitive at the end

That is why you get:

• Meine Schwester will langsam bohren.

Because langsam is being used as an adverb, not an adjective.

It describes how she wants to drill:

• langsam bohren = to drill slowly

Adverbs in German do not take adjective endings.

Compare:

• ein langsamer Bohrer = a slow drill/driller → adjective, with ending
• langsam bohren = to drill slowly → adverb, no ending

Because Meine Schwester is the subject of the clause, so it is in the nominative case.

For feminine singular, mein- takes:

• meine Schwester = nominative
• meiner Schwester = dative or genitive, depending on context

Here, your sister is the one doing the action, so nominative is required:

• Meine Schwester will ...

Because und is a coordinating conjunction. It joins two main clauses, and each main clause keeps normal verb-second word order.

So you have:

1. Meine Schwester will langsam bohren
2. ich halte die Wasserwaage an die Wand

In both clauses, the finite verb is in the second position:

• Meine Schwester
  • will
• ich
  • halte

If this were a subordinate clause introduced by something like weil, the verb would go to the end.

Die Wasserwaage is a spirit level or bubble level.

It is a compound noun:

• Wasser = water
• Waage = scale, balance

In German compounds, the last part usually determines the grammatical gender. Since die Waage is feminine, the compound is also feminine:

• die Wasserwaage

Because an is a two-way preposition. With these prepositions, German uses:

• accusative for direction / movement toward a place
• dative for location / position in a place

Here, the idea is that the speaker is holding or pressing the level up against the wall, so German uses accusative:

• an die Wand

Compare:

• Ich halte die Wasserwaage an die Wand. = I hold/put the level against the wall.
• Die Wasserwaage ist an der Wand. = The level is on/at the wall.

So the accusative here emphasizes contact directed toward the wall.

Because feminine singular nouns often have the same article form in the nominative and accusative.

For die Wand:

• nominative: die Wand
• accusative: die Wand

So even though Wand is the object of the preposition here, it still looks the same.

Compare that with a masculine noun, where the accusative often changes:

• nominative: der Tisch
• accusative: den Tisch

That is why English speakers sometimes miss the case here: the form does not visibly change.

Here, an is best understood as the preposition introducing die Wand:

• an die Wand

The main verb is halten.

So the structure is:

• ich halte
  • die Wasserwaage
    • an die Wand

This is not the same as the separable verb anhalten, which usually means things like to stop or to halt.

So in this sentence, think of it as:

• halten = to hold
• an die Wand = against/to the wall

Usually, no. In standard German, a comma is normally omitted before und when it connects two main clauses.

So many people would write:

• Meine Schwester will langsam bohren und ich halte die Wasserwaage an die Wand.

The version with the comma is not the most neutral standard choice here. A comma before und can sometimes appear for clarity, emphasis, or special sentence structure, but the normal default is no comma.

Yes, but langsam bohren is the most natural and straightforward placement here.

Because langsam modifies bohren, it fits well directly before the infinitive:

• Meine Schwester will langsam bohren.

Other placements are possible in broader contexts, but they may change the emphasis. For example, German word order often lets you move adverbs around to highlight different parts of the sentence.

So for a learner, the safest pattern is:

• modal verb + adverb + infinitive

which is exactly what you see here.