The co-ordinating conjunctions and and but are often used to combine clauses. They are placed between independent clauses to create compound sentences. Let us look at a few compound sentences with the two conjunctions.
Compare the following two sentences:
1a) It was hot yesterday, and today it is cold.
1b) It was hot yesterday, but today it is cold.
The meaning of the two sentences is a bit different. In (1b), the conjunction but expresses contrast. The speaker may be surprised that the weather has changed. However, in (1a), the speaker expresses no surprise.
The following sentences only differ in the use of the conjunction:
2a) I like tea, and she likes coffee.
2b) I like tea, but she likes coffee.
The first sentence expresses two preferences, but does not contrast them. In the second sentence, the use of the conjunction but emphasizes the difference between the two preferences.
The final pair of sentences again illustrates the difference.
3a) They like to have breakfast early, and they want to have breakfast at eight.
3b) They like to have breakfast early, but they want to have breakfast at eight.
In both sentences, it is clear that they like to have breakfast early. However, (3b) expresses a contrast. It appears that the speaker does not consider eight o'clock an early breakfast. In (3a), however, the speaker makes no judgement and may consider eight o'clock an early breakfast. In (3a), the speaker does not imply that eight is not an early breakfast, but in (3b), the speaker does.
Many compound sentences are formed with the conjunctions and and but. Though they can combine identical clauses, the meaning of the two sentences is not identical. The conjunction and is neutral about the meaning of the individual clauses, but but can be used by the speaker to highlight contrast, indicate surprise and also disagreement.