Die Übelkeit geht langsam weg, seit ich Tee trinke.

Übelkeit is a feminine noun, so its basic article is die.

In this sentence, die Übelkeit is the subject of the verb geht ... weg, so it is in the nominative case.

• die Übelkeit = nominative / accusative singular feminine
• Here it is nominative because the nausea is what is going away

If it were in a different role, the form around it could change, for example:

• wegen der Übelkeit = because of the nausea
• Ich habe die Übelkeit satt is not idiomatic, but here die Übelkeit would be an object

This is the separable verb weggehen.

In a normal main clause, separable verbs split:

• weggehen → geht ... weg

So:

• Die Übelkeit geht weg. = The nausea goes away.

The conjugated part geht takes second position, and the prefix weg moves to the end of the clause.

You will see the verb together again in other structures:

• weil die Übelkeit weggeht
• dass die Übelkeit weggeht
• Die Übelkeit kann weggehen.

Yes. It is very common and natural in everyday German.

German often uses weggehen for things like:

• pain
• nausea
• a headache
• fear
• a bad feeling

So Die Übelkeit geht weg sounds very normal.

Some alternatives are:

• Die Übelkeit verschwindet. = The nausea disappears.
• Die Übelkeit lässt nach. = The nausea is easing / getting less intense.

Compared with those, weggehen is especially everyday and conversational.

Yes, literally langsam means slowly, but in this sentence the most natural English idea is often gradually.

So Die Übelkeit geht langsam weg means the nausea is not disappearing all at once; it is easing little by little.

That is why English might translate it as:

• The nausea is slowly going away
• The nausea is gradually going away

Both ideas fit.

This is a very important German pattern.

After seit, when something started in the past and is still continuing now, German usually uses the present tense, not a past tense.

So:

• seit ich Tee trinke = since I started drinking tea / since I have been drinking tea

English often uses:

• since I started drinking tea
• since I’ve been drinking tea

But German normally says:

• seit ich Tee trinke

More examples:

• Ich wohne seit drei Jahren hier. = I have lived here for three years / I’ve been living here for three years.
• Seit er Sport macht, geht es ihm besser. = Since he started exercising, he has been feeling better.

Using seit ich Tee getrunken habe would usually sound wrong here, because the perfect tense suggests a completed action, not an ongoing habit or state.

Because seit introduces a subordinate clause here.

In German subordinate clauses, the conjugated verb goes to the end:

• seit ich Tee trinke

Compare:

• Main clause: Ich trinke Tee.
• Subordinate clause: seit ich Tee trinke

That is standard German word order after conjunctions like:

• weil
• dass
• wenn
• seit

So the structure is:

• Die Übelkeit geht langsam weg, seit ich Tee trinke.

Because Tee is being used as a general mass noun, like tea in English.

When German talks about drinking, eating, or using a substance in a general way, it often leaves out the article:

• Ich trinke Tee.
• Sie trinkt Wasser.
• Wir essen Brot.

You would use an article if you meant a specific tea:

• seit ich den Tee aus der Apotheke trinke
• seit ich einen Kräutertee trinke

So Tee without an article here just means tea in general.

Grammatically, seit is mainly about time here: since in the sense of from the time when.

So the basic meaning is:

• from the time I started drinking tea, the nausea has been going away

However, in real life, this can also suggest a connection or possible cause. A listener may understand:

• ever since I started drinking tea, the nausea has been improving

But seit itself is still primarily temporal, not purely causal.

If you want to state the reason more directly, German would usually use weil:

• Die Übelkeit geht langsam weg, weil ich Tee trinke.

That means because I drink tea, which is more directly causal.

Yes. Seitdem also works.

• Die Übelkeit geht langsam weg, seitdem ich Tee trinke.

This means basically the same thing.

In many contexts:

• seit = since
• seitdem = since / ever since

Seitdem can feel a little more explicit or a little more emphatic, but both are common and natural.

Yes, absolutely.

You can say:

• Seit ich Tee trinke, geht die Übelkeit langsam weg.

This is just as natural. German often allows this kind of clause order change.

When the subordinate clause comes first, the main clause still follows the normal rule that the conjugated verb comes early, so you get:

• Seit ich Tee trinke, geht ...

not

• Seit ich Tee trinke, die Übelkeit geht ...

So the first-position clause counts as one unit, and then the main verb geht comes next.