Basically, there are two ways to generate grammar exercises. The first is to define a lot of infinitely combinable building blocks. From those you create sentences to demonstrate or test a specific syntactic or morphological feature. This is what Duolingo is doing. Sentences like "A cat flew into the space" is a typical result of this approach.

The second way is to find complete sentences in authentic texts: books, newspapers, subtitles etc. You define a syntactic pattern, for example "ARTICLE + ADJECTIVE + NOUN" and search for a matching sentence in a textual corpus. That's what Fluentcards Grammar does.