English phrases or phrases in English language are grammar units (aka syntactic units). But aren't clauses too such units? Yes, but the phrase is a basic type of grammar unit; a clause is a higher level grammar unit.
Since we call the phrase the basic type of grammar unit, we need to understand the following things...
In this page, I'm going to deal with the first of these two questions.
When we call something a grammar unit, we mean that we look upon it through the eyes of grammar as being a single whole, whatever the number of parts it has.
We consider the phrase as a unit of grammar or syntax because it has a wholeness or unity in itself, though it may have many words in it.
If we get some groups of words in English, we cannot at once say whether they are English phrases or not. Just any random collection of words, for example, 'thoughts and in swam wow' is not a grammar unit.
Before we call something a phrase, we need to find out first if it is a grammar unit. How do we do that?
We are going to take a given phrase and see how it behaves.
We take the words, "four very simple questions" as our given phrase and we'll carefully watch its behaviour.
In the given phrase, we know that the word questions is the most important. It is called the head and the other three words serve it directly or indirectly.
The words four and simple serve the head word directly: 'four questions' and 'simple questions' are acceptable utterances.
The word very serves the head indirectly: 'very simple' is acceptable. The word very serves the word simple which in turn serves the word questions.
...the "outsider" is ready to serve the original phrase and blend with the words in it.
The words, "four very are simple questions" are NOT an English phrase because the word 'are' does not fit in but disturbs the unity. The word behaves like a pebble in your plate of rice.
On the other hand, four very simple grammar questions is a phrase because the word grammar acts as a modifier to questions and blends into the original phrase, like sugar in your cup of coffee.
Whatever disturbs the wholeness (unity) of the original phrase cannot be part of the phrase. It is rejected. Whatever can faithfully serve the wholeness of the original phrase is welcomed as part of it! How interesting! How human!
Grammar belongs to the depths of human existence. That's why we take it so much for granted!
Look at how our given phrase performs in different locations in a sentence: in the beginning, the middle, or the end.
But you can't move one part of the phrase to one place and another to a different location in a sentence.
Such breaking up of the phrase has destroyed the phrase as well as the sentences it was a part of.
In our three numbered sentences, it is possible to substitute our given phrase as...
The question, 'What is a phrase?' has also an alternative explanation. This is a traditional explanation and is based on the semantic function of a group of words in a sentence, rather than the syntactic composition of the word-group.