Ich nehme einen Kaffee.

Because Kaffee is masculine in German: der Kaffee.

In this sentence, Kaffee is the direct object of the verb nehmen, so it is in the accusative case. The masculine indefinite article changes like this:

• nominative: ein Kaffee
• accusative: einen Kaffee

So:

• Ein Kaffee ist teuer. = A coffee is expensive.
• Ich nehme einen Kaffee. = I’ll have a coffee.

Its basic meaning is to take, and that is still the core idea here.

But in everyday German, Ich nehme ... is also a very common way to say that you are choosing, ordering, or having something, especially in a café, restaurant, or shop.

So Ich nehme einen Kaffee is literally something like I take a coffee, but naturally it means:

• I’ll have a coffee
• I’ll take a coffee

It is in the accusative case because it is the thing being taken/chosen by the subject.

In Ich nehme einen Kaffee:

• Ich = the subject
• nehme = the verb
• einen Kaffee = the direct object

A lot of German verbs, including nehmen, take a direct object in the accusative.

You usually learn German nouns together with their article:

• der Kaffee = coffee

That der tells you the noun is masculine.

This matters because the article changes depending on gender and case. For example:

• der Kaffee → nominative
• einen Kaffee → accusative

So it is very important to memorize nouns as:

• der Kaffee
• not just Kaffee

The infinitive is nehmen = to take.

With ich, it becomes nehme:

• ich nehme = I take
• du nimmst = you take
• er/sie/es nimmt = he/she/it takes
• wir nehmen = we take
• ihr nehmt = you all take
• sie/Sie nehmen = they/you (formal) take

So Ich nehme is just the normal 1st person singular form.

Yes, very natural.

Ich nehme einen Kaffee is a normal and idiomatic way to say you are choosing or ordering a coffee.

Depending on the situation, German speakers might also say:

• Ich hätte gern einen Kaffee. = I would like a coffee.
• Für mich einen Kaffee, bitte. = A coffee for me, please.
• Einen Kaffee, bitte. = A coffee, please.

Compared with these, Ich nehme einen Kaffee can sound a little like I’ll take a coffee.

Yes, but the meaning is different.

• Ich nehme einen Kaffee = I’ll have / I’ll take a coffee
  
  • focuses on choosing or ordering
• Ich trinke einen Kaffee = I am drinking a coffee / I drink a coffee
  
  • focuses on actually drinking it

So if you are in a café and deciding what to order, Ich nehme einen Kaffee fits better.

Yes. Like ein, forms of the indefinite article can sometimes overlap with the number one.

In Ich nehme einen Kaffee, einen usually just means a.

If you want to stress the number, context or emphasis does that:

• Ich nehme einen Kaffee, nicht zwei. = I’ll take one coffee, not two.

So grammatically it is the same form, but the intended meaning depends on context.

This is standard German main-clause word order:

• Ich = subject
• nehme = verb
• einen Kaffee = object

German main clauses usually put the finite verb in second position. Here, the subject is first, so the verb comes second:

• Ich | nehme | einen Kaffee

You could also move another element to the front, but the verb still stays second:

• Einen Kaffee nehme ich.

That version is possible, but it sounds more marked or contrastive. The basic neutral sentence is Ich nehme einen Kaffee.

Usually, if you mean a coffee as one drink/order, you would say einen Kaffee.

Ich nehme Kaffee is possible, but it usually sounds more general, more like:

• I’ll have coffee
• I choose coffee
• I drink coffee

So:

• Ich nehme einen Kaffee. = one coffee / a coffee
• Ich nehme Kaffee. = coffee in general

In many ordering situations, einen Kaffee is the more natural choice.