Questions & Answers about در گرمترین صحرا هیچ آبی نیست.
Why does garm-tarin come before the noun, and why isn't there an Ezafe connecting them?
This is the special rule for superlative adjectives (the "-est" words). While normal adjectives come after the noun and connect with an Ezafe (like sahra-ye garm for "hot desert"), superlatives ending in -tarin come before the noun, and no Ezafe is used at all. So it is simply garm-tarin sahra.
Does aabi mean "blue" in this sentence?
No! While aabi is the Farsi word for "blue", here it is actually the word aab (water) plus the indefinite ending -i. When you use hich (no/any) with a noun, you almost always add this -i to the end of the noun. So hich aabi means "no water" or "not any water".
Why is the verb negative (nist) when we already used the negative word hich?
In Farsi, when you use words like hich (no/any/none), the verb must also be negative. It is similar to a "double negative" in English. You are literally saying "In the hottest desert, any water isn't," which is the standard and required way to say "there is no water."
How would the word order change if I just wanted to say "in a hot desert" instead of "the hottest desert"?
You would go back to the normal adjective rules: the adjective goes after the noun, and you use an Ezafe. You would say (in a hot desert).