Questions & Answers about Minun täytyy ostaa halvin varuste.
How do we build the superlative halvin from the basic adjective halpa (cheap)?
To add the superlative suffix -in, two things happen to the word halpa. First, the suffix closes the final syllable, which triggers consonant gradation and weakens the p to a v. Second, the final -a drops out to make room for the -i. This transforms halpa into halvin.
Why doesn't the object have an -n ending here (like halvimman varusteen)?
Normally, a singular total object gets an -n ending (the genitive-accusative case) to show that an action is completed. However, there is a special rule for necessity verbs like täytyy (must): their singular total objects always stay in the basic dictionary form (the nominative case).
Why is the word for "I" in the genitive case (minun) instead of minä?
Sentences that express necessity using verbs like täytyy or pitää always require their subject to be in the genitive case (ending in -n). You can think of the sentence literally translating to "Of me is necessary to buy..."