Context-free grammar

A context-free grammar is like a set of rules to make sentences. Imagine you have a bunch of Legos and you want to build a house. You can only use certain pieces in certain ways to make the house. Similarly, in grammar, you can only use certain words in certain ways to make a sentence.

But, in a context-free grammar, you don't have to worry about the context or meaning of the sentence, just the structure. It's like a game where you have to put words in order to make a sentence, but you can't use just any words, only the ones that fit the rules.

These rules usually take the form of a starting symbol (like a Lego instruction) and then a set of rules that tell you how to transform that symbol into other symbols until you end up with a sentence.

For example, if the starting symbol was "S" and the rules were:

S -> NP VP
NP -> Det N
VP -> V NP

Then you could create the sentence "The cat sleeps" by starting with S and using the rules to transform it into:

S -> NP VP -> Det N V NP -> The cat sleeps

So a context-free grammar is like a set of rules for making sentences by putting words together in a certain way, like building a Lego house.