Learn Farsi: هرچند حرارت زیاد است، طعم قهوه ترش است. - Even though the heat is high, the coffee's flavor is sour.

هرچند can take either the indicative or the subjunctive. Because this sentence states a known, concrete fact (the heat is actually high right now), it uses the indicative است. If it were a hypothetical situation (even if the heat were high), you would use the subjunctive.

Could I use اگرچه instead of هرچند at the beginning of this sentence?

Yes, اگرچه and هرچند are perfectly interchangeable here. Both mean although or even though, and both can be followed by the indicative to state a factual contrast.

How do we know that طعم and قهوه belong together as the coffee's flavor?

They are connected by the Ezafe, an unseen e sound that links a noun to a possessor or an adjective. You pronounce it as طعم قهوه (ta'm-e qahve). Because it is a short vowel, it isn't normally written in the standard Farsi alphabet.

Can I use حرارت to talk about the weather being hot?

While حرارت means heat, it is usually used for the physical heat of an object, like a stove, a body, or the brewing water for coffee. To talk about hot weather, you would usually just use the adjective گرم (warm/hot) or the word دما for temperature.