I answered in the imperative (or tried to) and was marked wrong. Should "Imigh leat" be correct in this sense?
I'm struggling with broad and slender. The verb to go (téigh) looks slender but the answer is 'téann tú' so the 'ann' ending indicates that its broad.
Words aren't broad or slender. Letters are.
For a 1st conjugation verb like téigh, the Tips & Notes say that you have ann as the "broad ending", and eann as the "slender ending" in the present tense, but that really only applies to a stem that ends in a consonant, like ól or dún or ith or bris - you have to use the correct ending based on that final consonant - you can't put ann after ith or bris because of the slender i before the final consonant, and you can't put eann after ól or dún because of the broad ó and ú before the final consonant. If the final consonant is broad (because there is a broad vowel before it), you use the "broad ending", if the final consonant is slender (because there is a slender vowel before it), you use the "slender ending".
But in the case of téigh, the stem is té, and you don't have a final consonant that needs to be kept either broad or slender, and there is no rule preventing a slender vowel é coming next to a broad a, so you get téann rather than téeann.