Learn Turkish: Kapının kilidi yeni. - The door's lock is new.

The word for door is kapı. To make it "the door's" (the genitive case), we add the suffix -ın. However, since kapı ends in a vowel and the suffix starts with a vowel, we must use a buffer letter. For the genitive case, that buffer letter is always -n-, making it kapının.

The dictionary says "lock" is kilit. Why is it spelled kilidi here?

This is an example of consonant mutation. When you add a suffix starting with a vowel (like the possessive -i here) to a word ending in p, ç, t, or k, that final letter usually softens. Here, the hard t in kilit softens to a d, becoming kilidi.

Why do we need the -i at the end of kilidi? Doesn't kapının already mean "the door's"?

In Turkish definite noun compounds, both words must have a suffix to link them together. The owner gets the genitive suffix (-nın) and the owned item gets the possessive suffix (-i). Even though English only marks the owner (with " 's "), Turkish requires both parts to be marked.

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