"Feumaidh tu bonaid."
Translation:You need a bonnet.
Scratch what I said! I looked it up, and found this. Does this sound more like it? "There are two forms for the 2nd person singular form (thu and 'tu), tu is used after verbs in the relative future tense and conditional mood."  -- https://gaelicgrammar.org/~gaelic/mediawiki/index.php/Personal_Pronouns#Basic_Grade
Relative (Independent) Future Tense is a verb that is being done in the future without verbal particle (A/an/am or Cha/Chan or Nach). Basically, a positive verb statement for something in the future. e.g. I will eat.
Conditional mood (simplistically, a mood is another type of tense - I will get shot for writing that!!!) is something that could happen. In English, we use could to express it. E.g. I could eat.
See Gaelic in 12 Weeks (OMaolalaigh) pages 100-102, 135-136, 152:
Use tu instead of thu after ---
• after verb forms ending in: -idh, -aidh (future independent forms)
• after verb forms ending in: -as, -eas (relative future forms)
• after verb forms ending in: -adh, -eadh (conditional, past habitual forms)