Question Words: ποιος, τι, πόσος

The three everyday question words reuse grammar you already have. ποιος (“who / which”) and πόσος (“how much / how many”) are adjectives: they agree with their noun in gender, number and case, declining on the -ος / -η / -ο pattern - ποιος δάσκαλος;, ποια μητέρα;, ποιο παιδί;, πόσος καφές;, πόση δουλειά;, πόσα παιδιά;. Note that ποιος takes the vowel-stem feminine -α (ποια, like νέα) while πόσος takes the ordinary πόση. τι (“what”) is the odd one out: it is completely invariable, one shape for every gender, number and case. So of the three, only τι never changes - and only τι can stand in front of a verb without agreeing with anything at all.