Questions & Answers about Çantaya koymam gereken ıslak mendil nerede?
How does koymam gereken literally mean 'that I must put'?
This is the standard way to form a relative clause of necessity. It combines the verbal noun koyma with the 1st person possessive -m (meaning 'my putting') and gereken, the subject participle of gerek ('which is necessary'). Literally, it translates to 'the wet wipe for which my putting is necessary'.
Why does çanta take the dative suffix (çantaya) when the translation says 'in the bag'?
In English, we focus on the final static location ('in'), but in Turkish, the verb koymak (to put) is treated as an action of motion toward a destination. Therefore, the destination takes the dative case (-y-a, 'to/into') rather than the locative case (-da, 'in/at').
Could I use the necessity suffix -malı to say this instead?
Yes, you could technically say koymalı olduğum ıslak mendil. This uses the -malı suffix plus the object relative clause olduğum (literally: 'the wet wipe that I am in a state of having to put'). However, the koymam gereken structure you see here is much more natural and common in everyday Turkish.
If I were asking about multiple wet wipes, would gereken take a plural suffix?