Questions & Answers about Minun täytyy pakata uusi vaate reppuun.
Why does reppu become reppuun?
This is the illative case, which expresses movement 'into' something. To form it, you double the last vowel of the word and add -n (often called the -Vn ending). So, reppu (backpack) becomes reppuun (into the backpack). If you wanted to ask 'Into where must I pack?', you would use the question word Mihin?
Why does the sentence start with Minun instead of Minä?
The verb täytyy (must) is a special necessity verb. In Finnish, the person who 'must' do something is put into the genitive case. So instead of the basic word minä (I), you have to use minun. Literally, the sentence structure is something like 'Of me it is necessary to pack...'
Why is the verb pakata not conjugated?
When you use a necessity verb like täytyy, it acts as the main conjugated verb. The action you actually have to do (in this case, packing) acts as the second verb in the sentence, so it stays in its basic dictionary form, which is called the infinitive.
Why are uusi and vaate in their basic forms without an object ending?
Usually, when you do something to a complete object, it gets an -n ending. However, there is a special exception for necessity verbs like täytyy: their objects never take the -n ending. Because of this rule, the object stays in its basic form (the nominative case) as uusi vaate.