Learn Turkish: Mucize süpürgeyi yarın alacaktım. - I was going to buy the miracle vacuum cleaner tomorrow.

This creates the "future in the past" tense, which translates to "was going to". You combine the future tense suffix -acak, the past tense suffix -tı, and the personal ending -m. Here, al (buy) + acak + tı + m means "I was going to buy". It usually implies a plan that was interrupted, changed, or didn't happen.

Why is it süpürgeyi instead of just süpürge?

The suffix -i is the accusative case ending, with -y- acting as a buffer letter between two vowels. Because you are talking about a specific item ("the" miracle vacuum cleaner) that is receiving the action of the verb, you must use the accusative case to mark it as a specific direct object.

Mucize is a noun meaning "miracle". Why isn't this a noun compound like mucize süpürgesi?

While mucize is technically a noun, it frequently acts as an adjective in everyday Turkish when describing amazing products, just like in English (e.g., "miracle cure"). Because it is treated as an adjective modifying süpürge here, they don't link together with the noun compound suffix -si.

Isn't it a contradiction to use a past tense verb like alacaktım with a future time word like yarın (tomorrow)?