grammar - Subject after "but"

"I love fruits but I am allergic to strawberries"

"They have bought a house in Manchester but they still haven't sold their house in London"

Does it really need to use "I am" & "they" after "but" in the above examples?

Can we leave the "I am" and "they" in the above examples and say,

"I love fruits but allergic to strawberries"

"They have bought a house in Manchester but still haven't sold their house in London"

I want to know the grammar rules behind this, that when we can leave the subject or subject+verb after "but".

This doubt really came to my mind when I was writing something like this

"I don't sleep at night but in the morning"

I left the words "I sleep" after "but" in the above example.

Should that be -

"I don't sleep at night but I sleep in the morning"

But it looks redundant to me.

Please help me with this.

Answer

You can avoid repeating the Subject after But.

Coordinating conjunctions (such as 'But', 'And') are defined as:

Conjunctions placed between words, phrases, clauses, or sentences of equal rank.

When the Subject in a sentence is same for two verbs, then, using 'But' would make a Compound Verb, and its fine not to repeat the subject in such cases.

Therefore, your sentence can be written as:

I don't sleep at night but in the morning.

(Edit- answered the ques asked in the comment section):

Since the verb is one and the same it can be omitted alongwith the subject (after using the coordinating conjunction).

But in cases where the verb is different, it cannot be omitted. For example:

I came late but was excused.

For further insight, you can have a look at this: https://english.stackexchange.com/questions/233741/do-i-have-to-repeat-the-same-subject-after-and-when-introducing-a-new-verb

The below mentioned link would give you the understanding of when to use a Comma before But, in such cases: https://www.grammarly.com/blog/comma/ (13th Point)