"You need another glass."
Translation:Feumaidh tu glainne eile.
The pronoun originally had unlenited t – tú in Old Irish, cognate with English thou – then in Scotland it got lenited in most cases to /þu/ (thu with th like in English thanks), then /þ/ changed to /h/ and hence the modern thu /hu/.
But lenition tends to be blocked when like consonants (homoorganic ones to be precise, ie. such that are produced in the same place in the mouth) stand next to each other, and at that times when th was still /þ/, dh was /ð/ (voiced th as in there) – and all /t/, /d/, /þ/ and /ð/ were dental consonants. The similarity between them caused the t to not get lenited after dh, so it stayed /t/ to modern times.
That’s the historical reason.
The modern rule would be to just use tu instead of thu when verbs ending in -idh, -dh, -n or -s stand before it. Wikipedia describes it thus:
In most cases the Classical Gaelic lenited form of tu, i.e. thu, has become generalised. Tu is retained in constructions where it is preceded by a verb ending in -n -s or -dh (incl. historic -dh):
Is tu a rinn a' mhocheirigh! "You are an early riser!"
Bu tu an gaisgeach! "What a hero you were!" (In older Gaelic bu was written and pronounced budh)
Mun abradh tu “deas-dé.” "Before you had time to say even a single word."
Cuiridh tu an-seo e! "You will put it here!"