Learn Turkish: Büyük ev sıcak mı?

The question particle changes its vowel to match the last vowel of the word right before it. This is called 4-way vowel harmony. The last vowel in sıcak is a. Because a is a back vowel, the particle must use the matching back vowel ı, making it mı.

If mı follows vowel harmony like an ending, why is it written as a separate word?

That is a special rule for the yes/no question particle! Even though it behaves like a suffix by looking at the previous word (sıcak) to decide its vowel, it is always written with a space before it.

Where is the word for 'is' in this sentence?

Turkish doesn't have a direct translation for the verb 'to be' (am/is/are) in the present tense. You just place the subject (Büyük ev, meaning 'The big house') next to the descriptive word (sıcak, meaning 'hot'). Adding mı at the end turns that simple statement into the question 'Is the big house hot?'.

How do we know it means 'Is the big house hot?' and not 'Is the hot house big?'?