Questions & Answers about Утром я пил горячий кофе.
Why use the imperfective verb пил here instead of the perfective выпил?
Because the sentence focuses on the ongoing process of drinking ('was drinking') rather than a completed result ('drank it all'). When we talk about an action in progress in the past, we use the imperfective aspect.
Why does the adjective горячий have a masculine ending when кофе ends in -е, which is usually neuter?
Why does the sentence start with утром instead of just утро?
The dictionary word утро just means 'morning'. To say 'in the morning', Russian changes the word to утром (this is the instrumental case). This turns it into a time word answering the question 'when?'.
Why don't the words горячий кофе change their endings, since they are the direct object of the verb?
They are in the accusative case, but you cannot see a change! Кофе is a borrowed foreign word that never changes its spelling. For inanimate masculine nouns, the accusative adjective ending is identical to the dictionary form, so горячий stays exactly the same too.