Macmillan pioneered the concept of "Red Words." In their online ecosystem, the most common 7,500 words of English are marked in red. When you search a collocation, you immediately see which combinations are essential for everyday speech versus which are for academic essays. The online dictionary filters results by frequency, saving you from memorizing rare or archaic phrases.
In the quest for English fluency, most learners focus on two things: vocabulary and grammar. You learn that "fast" means quick, and "car" is a vehicle. But when you try to say "fast car," you are already using a —a natural combination of words that native speakers use. macmillan collocations dictionary online