正 (zhèng) has two main grammatical roles. As an adverb it means just at this moment or right in the middle of doing something, typically appearing before a verb (正在做 means is in the process of doing; 正好 means just right or exactly). As an adjective it means upright, correct, or main/primary - contrasting with 副 (fù, secondary). The compound 正在 is one of the most common progressive aspect markers in Mandarin.