Why writing “but” after period is wrong?

Because “but” is a coordinating conjunction which links coordinated sentences. Period marks newly created sentences, where the new sentence is syntactically unlinked. Thus, starting “but” after a period is contradictory to the syntax of the period.

What is a coordinating conjunction?

A conjunction is a word that marks connections between sentences or parts of sentences that work independently as sentences. Coordinating conjunctions allow for the deletion or “ellipsis” of part of the sentence while implying the rest of it.

Examples given by ChatGPT:

And (adds information)

Full: She read the book, and she wrote a review.

Ellipsis: She read the book, and (she) wrote a review.

Nor (negative alternative)

Full: He didn’t call me, nor did he text me.

Ellipsis: He didn’t call me, nor (did he) text me.

But (contrast)

Full: Today’s assignment looks easy, but it is difficult.

Ellipsis: Today’s assignment looks easy, but (it) is difficult.

Or (choice)

Full: You can drive, or you can take the bus.

Ellipsis: You can drive, or (you can) take the bus.

Yet (contrast, like “but”)

Full: Sally tried to do it, yet she got tired soon.

Ellipsis: Sally tried to do it yet (she) got tired soon.

How this relates to the period?

The period marks the end of a sentence that works individually as a unit. However, in the above sentences, each sentence in the string may be substituted by a period:

She read the book, and she wrote a review. -> She read the book. She wrote a review.

• He didn’t call me, nor did he text me. -> He didn’t call me. He did not text me.
• Today’s assignment looks easy, but it is difficult. -> Today’s assignment looks easy. It is difficult.
• You can drive, or you can take the bus. -> You can drive. You can take a bus.
• Sally tried to do it, yet she got tired soon. -> Sally tried to do it. She got tired soon.

With the period you can still make well-formed sentences, yet, why the feeling that something is missing? Because something is missing. Just that there’s nothing missing in the “syntax” realm. It is a semantic connection between the ideas. In that sense, coordinating conjunctions are merely rhetoric devices out of the syntax.

The connectors

Connectors are conjunctions of larger chunks of sentences. They serve to the same purpose of the coordinating conjunction, but after ellipsis the connection with a previously stated subject is weaker:

• He didn’t call me, nor did he text me. -> He didn’t call me. Also, did not text me. ❓
• Today’s assignment looks easy, but it is difficult. -> Today’s assignment looks easy. However, is difficult.❌
• You can drive, or you can take the bus. -> You can drive. Also, can take a bus. ❌
• Sally tried to do it, yet she got tired soon. -> Sally tried to do it. However, got tired soon. ❓

The oddity of the second sentences is they leave you wondering if the action refers back to the person or item stated in the first subject, or to someone or something else mentioned in a paragraph.

Back to the “but” problem

Evidence that “but” after period is wrong could be shown in the inability to allow for ellipsis:

• Today’s assignment looked easy, but gave me trouble. ✅ VERSUS Today’s assignment looked easy. But gave me trouble. ❌

More examples provided by ChatGPT:

Ellipsis inside one sentence (natural)

• Today’s assignment looked easy, but gave me trouble. ✅
• The hike was long, but (it was) worth it. ✅
• He wanted to join, but (he) couldn’t. ✅
• The idea sounds strange, but (it) makes sense. ✅
• She seems shy, but (she) sounds confident in class. ✅

After a period (odd/fragment-like)

• Today’s assignment looked easy. But gave me trouble. ❌
• The hike was long. But was worth it. ❌
• He wanted to join. But couldn’t. ❌
• The idea sounds strange. But makes sense. ❌
• She seems shy. But is confident in class. ❌

Why “but” sounds good to me when I use it after period?

Because it is good. As I said, both “but” and “however” mean the same, and they are just rhetorical devices, outside of syntax. Just that “but” implies a higher degree of intimacy between the sentences linked. There may be degrees of intimacy in a connective continuum between conjunctions and connectors. One revealing example is the conjunction “so”, which is supposed to be a coordinating conjunction, but does not allow for ellipsis.

• I went to the store so I could retain the clients ✅ I went to the store, but I couldn’t retain the clients ✅
• I went to the store so could keep the clients. ❌ I went to the store but couldn’t keep the clients.✅

The continuum in connecting particles may look like this:

• However = full independence. Separates independent sentences. Does not allow the deletion of a repeated subject. Fully coherent with a previous period. Connecting conjunction.
• So = some independence. Separates coordinated sentences. Does not allow the deletion of a repeated subject. Mostly coherent with a previous period. Coordinating conjunction.
• But = some dependence. Separates coordinated sentences. Allows the deletion of a repeated subject. Less coherent with a previous period. Coordinating conjunction,
• That = complete dependence. Signals the presence of an embedded sentence. Impossible to separate. Not coherent with a previous period. “I didn’t mention that you wouldn’t go”. Subordinating conjunction.

This is just an example of how the continuum functions, but there would be more items within a larger spectrum. The issue is that the traditional linguistic framework, based on the generativist assumption, typically classifies grammatical items in a binary +/- paradigm. Generativism influenced language ideologies that see correction as absolute. In contrast, a Functionalist perspective recognizes degrees of attachment and detachment, not absolutes.

Then, is it finally OK, or is it wrong?

This article provided reasons to consider “but” incorrect after a period, for those who want to sustain its prohibition. However, to make it sound right in a text, the sentence after “but” should be short and add drama to what you said previously. But it’s up to you.

References:

ChatGPT (Aug. 16, 2025). Conversation with Ana Diaz. Open AI. Retrievable from: https://chatgpt.com/share/68a12534-bcec-8008-9cf8-33868b4dd9bf