Auf dem Tisch ist ein kleiner Kratzer, aber er stört mich nicht.

Because auf is a two-way preposition in German. That means it can take either:

• dative for a location: where something is
• accusative for a direction/destination: where something is going

Here, the scratch is already located on the table, so German uses the dative:

• auf dem Tisch = on the table

Compare:

• Der Kratzer ist auf dem Tisch. = The scratch is on the table.
• Ich lege das Buch auf den Tisch. = I put the book onto the table.

So in your sentence, dem shows location, not movement.

Because Tisch is a masculine noun: der Tisch.

When a masculine noun is in the dative singular, der changes to dem:

• nominative: der Tisch
• dative: dem Tisch

So:

• auf dem Tisch

is simply the correct dative form after auf when it describes location.

Both are possible, but the version with Auf dem Tisch first is very natural in German.

German main clauses follow the verb-second rule: the finite verb must be in the second position. The first position can be taken by different elements, not just the subject.

So you can say:

• Auf dem Tisch ist ein kleiner Kratzer.
• Ein kleiner Kratzer ist auf dem Tisch.

Both are grammatical.

The first version puts a little more focus on where the scratch is. This kind of word order is very common in German.

Because the adjective klein has to take the correct adjective ending.

Here, Kratzer is:

• masculine
• singular
• nominative
• used with the indefinite article ein

In that pattern, the adjective takes -er:

• ein kleiner Kratzer

This is one of the standard adjective ending patterns in German.

A few comparisons:

• ein kleiner Kratzer — masculine
• eine kleine Tasche — feminine
• ein kleines Buch — neuter

So kleiner is correct because it matches Kratzer.

Kratzer is masculine: der Kratzer.

Yes, in German you usually need to learn nouns together with their article:

• der Kratzer = scratch

That helps you use the correct forms later:

• ein kleiner Kratzer
• der Kratzer
• er for the scratch

Sometimes noun endings give hints, but they are not perfectly reliable. The safest habit is to memorize the noun and article together.

Because German pronouns follow grammatical gender, not natural gender.

Since der Kratzer is masculine, the pronoun is:

• er = he/it

So:

• der Kratzer ... er

Even though in English you would call a scratch it, German uses er because the noun is masculine.

A few examples:

• der Tisch → er
• die Lampe → sie
• das Buch → es

So er here means it, not he in the natural-gender sense.

The verb is stören, which means to bother or to disturb.

Its basic pattern is:

• etwas stört jemanden
• something bothers someone

So in your sentence:

• er = the scratch
• stört = bothers
• mich = me

That gives:

• er stört mich = it bothers me

Then nicht negates the verb phrase:

• er stört mich nicht = it does not bother me

So mich is in the accusative, because stören takes a direct object.

In German, nicht is often placed late in the clause, especially when it negates the whole idea of the verb phrase.

Here:

• er stört mich nicht

means he/it does not bother me.

Placing nicht at the end is very natural here because it negates the whole statement stört mich.

Compare:

• Er stört mich nicht. = He/It doesn’t bother me.
• Er stört nicht mich, sondern dich. = He/It bothers not me, but you.

In the second sentence, nicht comes earlier because it specifically contrasts mich.

Aber means but and connects two main clauses:

• Auf dem Tisch ist ein kleiner Kratzer
• aber er stört mich nicht

It introduces contrast:

• there is a scratch on the table,
• but it does not bother me.

A useful grammar point: after aber, German still uses normal main clause word order. So:

• aber er stört mich nicht

not:

• aber er mich nicht stört

That second version would be wrong in a normal main clause.

Yes, but it sounds a bit different.

German often prefers:

• Auf dem Tisch ist ein kleiner Kratzer.

This is a natural way to introduce something that exists in a place.

You may also hear:

• Es ist ein kleiner Kratzer auf dem Tisch.

But in many contexts, the version starting with Auf dem Tisch sounds more idiomatic and smoother.

So your sentence is a very typical German way to say it.

Yes. In this sentence, ein kleiner Kratzer is the subject, and subjects are in the nominative.

The structure is basically:

• Auf dem Tisch = location
• ist = is
• ein kleiner Kratzer = the thing that exists there

So ein kleiner Kratzer is nominative.

This is why you get:

• ein kleiner Kratzer
• not a dative or accusative form

Even though it comes after the verb, it is still the subject. German word order can move things around, but the grammatical role stays the same.