Sonst trinke ich morgens Kaffee, aber heute trinke ich Tee.

Here, sonst means normally, usually, or on other days.

It is showing a contrast:

• Sonst trinke ich morgens Kaffee = that is the speaker’s usual habit
• aber heute trinke ich Tee = today is different

A useful thing to know: sonst can also mean otherwise in other sentences, but in this one it is about the speaker’s normal routine.

Because German main clauses usually follow the verb-second rule.

In Sonst trinke ich morgens Kaffee, the first position is occupied by Sonst, so the conjugated verb trinke must come next:

• Sonst = position 1
• trinke = position 2
• ich = after the verb

So this is normal German word order.

If the sentence began with Ich, then you would get:

• Ich trinke sonst morgens Kaffee

That is also possible, but it puts the focus in a slightly different place.

morgens means in the mornings or every morning / generally in the morning.

That final -s matters.

Compare:

• morgens = in the mornings, habitually
• morgen = tomorrow
• heute Morgen = this morning

So in this sentence, morgens fits because the speaker is describing a usual habit, not talking about tomorrow.

In German, with food and drinks, it is very common to leave out the article when speaking in a general way:

• Ich trinke Kaffee
• Sie isst Brot
• Wir trinken Tee

This sounds natural when you mean the substance in general, not one specific item.

If you add an article, the meaning changes slightly:

• Ich trinke einen Kaffee often means I’m having a coffee
• Ich trinke den Kaffee means I’m drinking the specific coffee

So in your sentence, no article is completely normal.

Because heute has been placed at the front for emphasis or contrast.

German still keeps the verb in second position:

• heute = position 1
• trinke = position 2
• ich = after the verb

So aber heute trinke ich Tee highlights today as the important contrast.

You could also say:

• aber ich trinke heute Tee

That is also correct, but it sounds a little less contrastive. The original version strongly highlights that today is the exception.

No. Aber does not change the basic word order of the clause that follows.

After aber, you still have a normal main clause with the verb in second position:

• aber heute trinke ich Tee

So it is not:

• position 1 = aber
• position 2 = heute

That is not how German counts it. Aber connects the clause, but the clause itself still follows normal main-clause word order.

Also, the comma before aber is correct, because it joins two main clauses.

Because the second part is a full new clause:

• Sonst trinke ich morgens Kaffee
• aber heute trinke ich Tee

In English, learners sometimes want to think of this as Normally I drink coffee in the morning, but today tea, but German normally wants the full clause with the verb included.

So repeating trinke ich is the natural way to say it.

Yes, it is all in the present tense: trinke.

German present tense is used for both:

• habitual actions: I drink coffee in the mornings
• current or situation-based actions: today I’m drinking tea

The time words tell you how to understand it:

• sonst makes it habitual
• heute makes it about today

So the same tense works for both ideas.

Because they are nouns, and in German all nouns are capitalized.

So:

• Kaffee
• Tee
• Morgen when it is a noun, as in heute Morgen

This is one of the most noticeable spelling differences between German and English.

Yes, absolutely.

Normalerweise trinke ich morgens Kaffee, aber heute trinke ich Tee is a very natural alternative.

The difference is mostly nuance:

• sonst often contrasts the normal situation with the current exception
• normalerweise explicitly means normally / usually

For many learners, normalerweise feels a bit more transparent at first, while sonst is shorter and very idiomatic. Both work well here.