Quick brown fox jumps over lazy dog.
The quick brown dream jumps over a lazy nightmare.
A lazy dog was involved in a jumping-related incident with a quick brown fox.
The problem in the first sentence is grammar. While it is acceptable headlinese, it is not standard English, which requires every singular countable noun to be preceded by a determiner: a/the/this/that/one/another/my/your etc fox and dog. It is, however, almost perfectly understandable.
The problem with the second sentence is meaning. It is perfectly grammatical. A quick dream is possible, and it might even be a brown one, but a nightmare really can’t be said to be lazy, even one of those in which you are walking very, very slowly. And then a dream can’t be said to jump over a nightmare (though just maybe a nightmare might be said to jump over a dream).
The problem with the third sentence (one of the transformations of the first sentence by Vijith Assar) is style. It is perfectly grammatical, but takes almost twice as long to say very little, among other things downgrading the vivid active verb ‘jump’ and not identifying exactly what jumped what.
Many language websites and blogs fail to distinguish between grammar, meaning and style. Even spelling and punctuation are lumped into one category called ‘grammar’ (Arnold Zwicky has coined ‘garmmra’, but it hasn’t caught on). Some sensible bloggers (for example Jonathan Owen [edit: who I first described as ‘seemingly inactive, unfortunately’, but who contacted me to say that he is just posting less often]) systematically dismantle online ‘grammar’ quizzes, but they proliferate anyway.
In addition to grammar, meaning and style, there is also spoken and written, and informal and formal language (Geoffrey Pullum instead divides language into formal and normal), and jargon, slang, new coinages and word-play. On top of that, language can also be public or private, and prepared or spontaneous. And then there’s native speaker English (subdivided first into ‘British’ and ‘North American’, second into however many national Englishes and third into however many regional varieties), ‘official language countries English’ (for example, India, Nigeria, Hong Kong, Singapore) and second language speaker or learner English, influenced by the native language of the speaker or learner.
Very rarely does one size fit all – there are just too many variables. We still manage to communicate, somehow, most of the time.