Tha tinneas an t-siùcair orm

Tha mi ag iarraidh: "I want"

Tha: "is" (used here as "am")

mi: "I"

ag iarraidh: "wanting" or "want"

donut ithe: "to eat a donut"

donut: "donut" (the same as English)

ithe: "to eat"

ach: "but"

tha tinneas an t-siùcair orm: "I'm diabetic" (literally " diabetes is on me")

tha: "is" (used here as "have")

tinneas: "illness" or "disease"

an t-siùcair: "of sugar" (diabetes is often referred to as "sugar illness")

orm: "on me" (used to indicate that someone has an illness)

So, the full sentence "Tha mi ag iarraidh donut ithe ach tha tinneas an t-siùcair orm" means "I want to eat a donut but I have diabetes."