Ho mal di gola, quindi oggi bevo solo tè tiepido.

Because Italian usually expresses many physical sensations with avere (to have), not essere (to be).

So:

• Ho mal di gola = I have a sore throat
• Ho fame = I’m hungry
• Ho sete = I’m thirsty
• Ho caldo = I’m hot

Using essere here would sound wrong in standard Italian.

Mal di gola is a fixed expression meaning sore throat.

It is built like this:

• mal = pain / ache / bad
• di = of
• gola = throat

So literally it is something like pain of the throat, but in natural English the meaning is simply a sore throat.

You will see the same pattern in other expressions:

• mal di testa = headache
• mal di schiena = backache
• mal di denti = toothache

Because mal is the shortened form of male used before certain words in set expressions.

Compare:

• Sto male = I feel unwell
• Ho mal di testa = I have a headache

So in mal di gola, Italian uses the shortened form mal as part of a fixed pattern.

After avere, the usual idiomatic form is avere mal di... with no article:

• Ho mal di gola
• Hai mal di testa?
• Abbiamo mal di stomaco

You can use il mal di gola when the expression is functioning as a noun phrase by itself:

• Il mal di gola è fastidioso. = A sore throat is annoying.
• Questo farmaco cura il mal di gola. = This medicine treats a sore throat.

But with avere, the most natural form is avere mal di gola.

Yes. Gola means throat.

It can be used literally:

• Mi fa male la gola. = My throat hurts.

In mal di gola, it is part of the common expression for sore throat.

A useful comparison:

• Ho mal di gola = I have a sore throat
• Mi fa male la gola = My throat hurts

Both are natural, but the first one is a very common fixed expression.

Quindi here means so, therefore, or thus.

It connects the two ideas:

• Ho mal di gola = I have a sore throat
• quindi oggi bevo solo tè tiepido = so today I’m only drinking lukewarm tea

It is a very common linking word in Italian.

Examples:

• Sono stanco, quindi vado a dormire. = I’m tired, so I’m going to sleep.
• Piove, quindi restiamo a casa. = It’s raining, so we’re staying home.

Italian often uses the present tense for something happening now, today, or as a current decision.

So:

• oggi bevo solo tè tiepido = today I’m drinking only lukewarm tea

This sounds natural even though it refers to today as a whole. Italian does not need a future tense here.

You could also say:

• oggi berrò solo tè tiepido

That is grammatical too, but bevo sounds very natural and immediate in everyday speech.

Yes, oggi is flexible.

In the sentence:

• Ho mal di gola, quindi oggi bevo solo tè tiepido

oggi modifies bevo and means today.

You could also say:

• Oggi bevo solo tè tiepido.
• Bevo solo tè tiepido oggi.

But the original position is very natural because it clearly sets the time frame right before the verb.

Here solo means only, and it modifies tè tiepido.

So the idea is:

• I drink only lukewarm tea

It means that lukewarm tea is the only thing the speaker is drinking.

Position matters:

• bevo solo tè tiepido = I drink only lukewarm tea
• bevo tè solo tiepido would suggest the tea is only lukewarm, but this is not the natural way to say it here

The original sentence is the normal wording.

Because tiepido agrees with tè, which is masculine singular in Italian:

• il tè tiepido = the lukewarm tea

Adjectives in Italian usually agree in gender and number with the noun:

• masculine singular: tiepido
• feminine singular: tiepida
• masculine plural: tiepidi
• feminine plural: tiepide

For example:

• una tisana tiepida = a lukewarm herbal tea
• due tè tiepidi = two lukewarm teas

Not exactly.

• tiepido = lukewarm
• caldo = hot / warm
• freddo = cold

So tè tiepido is tea that is not very hot. In this sentence, that makes sense because someone with a sore throat may prefer tea that is gentle and not too hot.

The accent distinguishes tè (tea) from te (you, after a preposition).

Compare:

• Bevo il tè. = I drink tea.
• Con te vengo anch’io. = I’m coming with you too.

So the accent is important for spelling and meaning.

Yes, in standard Italian tè is masculine:

• il tè
• un tè caldo
• questo tè

So in the sentence, tiepido is masculine because it agrees with tè.

The comma separates the two clauses and makes the logic clear:

• Ho mal di gola
• quindi oggi bevo solo tè tiepido

In English, this is similar to writing:

• I have a sore throat, so today I’m only drinking lukewarm tea.

The comma is very natural here, especially in writing, because quindi introduces the result or consequence of the first statement.