Learn Turkish: Dün izlediğim film ortalama. - The movie that I watched yesterday is average.

This is the core of the object relative participle! We take the verb root izle- (to watch), add the suffix -dik (which acts like "that" or "which"), and finally add the possessive suffix -im (my). So literally, it translates to "my watched" or "that I watched".

Why is there a "ğ" in izlediğim? The suffix is supposed to be -dik.

This is because of consonant mutation. When you attach the vowel-starting possessive suffix -im to the suffix -dik, the k softens into a ğ. So, izle + dik + im becomes izlediğim.

Does the -dik suffix specifically mean this happened in the past, since it says Dün (yesterday)?

The -dik suffix actually covers both the past and present tenses ("that I watched" or "that I watch"). It is the word Dün (yesterday) that clearly tells us this specific action happened in the past.

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

In Turkish, you don't always need a separate verb for "is" or "are". Here, ortalama (average) is the main point of the sentence. For the "it" form (the movie), the "to be" suffix is often just left blank, so ortalama by itself at the end of the sentence means "is average".