Antony J. Peck
It should be obvious that we cannot learn by heart all the sentences we will ever need to use in a foreign language. Even if we could learn by heart a very large number of sentences, we would still need to be able to have a way of deciding in which situations we could use them appropriately.
We can see how absurd it is to rely exclusively on learning by heart phrases and sentences if we take at random some isolated sentences from a well-known phrase book.
The wash-basin is clogged.
The food is cold.
That man keeps following me.
Where are our drinks?
I need an axe.
Where is the British Consulate?
The absurdity of learning lists of sentences like these is clear because we know that each sentence fits a predetermined situation which we may or may not encounter. Since we cannot predict the situations in which we will find ourselves in a foreign country or when talking to foreigners, much of what we may learn by heart will be useless.
Grammar gives us the means of making up our own sentences, and expressing our own meanings. Grammatical rules allow us to respond creatively to whichever situation we find ourselves in. Grammar frees us from the expectations of the phrase-book author and the school textbook author; it allows us to send our very own messages.