Learn Turkish: Sen horozdan çok korkuyordun. - You were very afraid of the rooster.

In English, you are afraid of something, but in Turkish, you are afraid from something. The verb korkmak (to be afraid) always requires the ablative case (-dan / -den). So horozdan literally translates as from the rooster.

How does the ending of korkuyordun build the meaning 'were afraid'?

This combines two tense markers. First, we add the continuous marker -uyor to the root kork-, which gives the idea of an ongoing action. Then, we add the past tense marker -du and the 'you' ending -n. Together, -uyordun creates the past continuous tense, meaning 'you were fearing' or 'you used to fear'.

Is the word Sen absolutely necessary at the beginning?

No, you can drop it. The -n at the very end of korkuyordun already shows that 'you' are the subject. Including Sen just adds emphasis, similar to pointing out that 'You were the one who was very afraid.'

Why is çok placed right before the verb?

In Turkish word order, adverbs like çok (meaning 'a lot' or 'very') usually go right before the verb they describe. Since it describes the degree of your fear, it goes immediately before korkuyordun.