Progressive theory, also known as the aspect of continuity, is an essential concept in Bulgarian grammar. It refers to the expression of an ongoing action or state. In Bulgarian, the progressive aspect is formed by combining the auxiliary verb ‘съм’ (am) with the present participle of the main verb.

For example, in the sentence “Аз чета книга” (I read a book), when we want to express that this action is happening at this very moment, we use the progressive aspect and say “Аз съм четейки книга” (I am reading a book).

The progressive aspect is also commonly used to express future actions or plans. For instance, the sentence “Тя ще пътува утре” (She will travel tomorrow) can be transformed to the progressive form as “Тя ще бъде пътуваща утре” (She will be traveling tomorrow).

Overall, the progressive theory is a fundamental aspect of Bulgarian grammar that allows us to express ongoing actions or states in a precise and accurate manner.