Learn Farsi: نه، کلید داخل ماشین است.

You would add the question word آیا (âyâ) to the very beginning of the statement: آیا کلید داخل ماشین است؟ (Âyâ kelid dâkhel-e mâshin ast?). In everyday speech, Iranians often drop آیا and just raise their voice at the end of the sentence to ask the same thing.

How do we link the words for 'inside' (داخل) and 'car' (ماشین) when speaking?

They are connected by a short 'e' sound called the Ezâfe. You pronounce it as داخلِ ماشین (dâkhel-e mâshin). This hidden vowel connects nouns to their locations or descriptions, working a bit like the English word 'of' (inside of the car).

The English translation says 'the key' and 'the car', but where is the word for 'the' in Farsi?

Farsi does not have a direct translation for the definite article 'the'. A noun like کلید (kelid) can mean 'a key' or 'the key' depending entirely on the context. Since the speaker is answering a question about a specific key, we know to translate it as 'the key'.

The sentence ends with است (ast) for 'is'. Is this how it sounds in casual conversation?

Not usually. است (ast) is the formal, written form. In casual spoken Farsi, it is almost always shortened to an 'e' sound attached to the end of the previous word. A native speaker would likely say ماشینه (mâshin-e) instead of ماشین است (mâshin ast).