1
When we change the subject to which we are refering to in the sentence, we add a comma.
“I am here, you are there.”
To check, search for the nouns and, if you see several, identify the parts in which each of them is subject.
We do it because it is useful for the reader to know when you start talking about someone or something else.
1.1
A conjunction (for, and, nor, but, or, yet, and so) can be placed after the comma in the first case. Separating two clauses with comma (as opposed to a full stop) indicates that they are logically connected, adding a conjuction makes the type of the connection precise, so always add one, unless your intention is too keep an air of mystery around it.
“I am here yet you are there.”
Almost forgot - you can omit the comma before the conjuction, but only if the clauses are very short and it’s clear which is which (e.g. so short that, you can read the sencence without making a pause.)
2
When we want to add a word or a phrase to a grammatically sound sentence we enclose that in commas. In most instances this can be viewed as an instance to the previous rule, because the added word or phrase would almost always have a different subject, but it is so important that I put it on it’s own.
“I am here, you, and the person you are with, are there.”
The other common case of this is when you are adding some metacommentarty:
“I am here, you, however, are there.”
To check, remore the text that you plan to enclose in commas and see if the sentence is still gramatically correct.
2.1
When we want to add a word or a phrase to a grammatically and logically complete sentence, we may enclose that in brackets instead of commas.
“I am here, John (the person who rides a bicycle) is there.”
To check, remore the text that you plan to enclosed in brackets/dashes and see if the sentence is still logically correct.
Again, in order for you to do that, the sentence must make sense without the text in brackets. For example, replacing commas with brackets here would be inappropriate:
“I am here, John, the person who rides a bicycle, is pedalling through the slope.”
3
That and which. The difference between the two is similar to the difference between separating by commas and separating by brackets by the rule above - use which when we use brackets and that when we use commas i.e. when the qualification is needed to understand the sentence.
“Give me the pen that can be found on my desk.” “Give me the pen which I need for my project.”
People often confuse this rule, because it is very context-dependent, e.g. you can say.
“Give me the pen that I need for my project.”
But it would mean that the qualification is somehow significant for the request i.e. that you have several pens and you want to communicate that you want the one pen that you need for your project go you are saying the second part to prevent him from giving you the wrong pen.
4
When listing things, use commas to separate them.
I like trees, pushups and forests.
No comma before the last “and” because “and” as per rule 1.1, but only if it is abundantly clear what the context is e.g. here it is not:
I like climbing trees, doing pushups, and forests.
This rule is known as the oxford comma.
To check, see if the sentence can be read in a different way e.g. in the example above
5
i.e. and e.g. Both are used in front of lists of things but i.e. means “in other words”, e.g. means “for example”. Use i.e. when the list is complete, e.g. when it’s not. “Water is necessary for sustaining the life of all types of living organisms i.e. bacteria, animals and plats.”
Water is necessary for sustaining the life of all types of living organisms e.g bacteria.
6
No references between sentences, use a comma to connect two thoughts that are related. and a full stop if they are not.
6.1
Absolutely no references between paragraphs and they should If a given sentence contains any reference to the previous sentence (e.g. if you say “they” refering to a person or people that you talk about in the previous sentence) then those two sentences belong in one paragraph.
I like my brother. He always brings me cookies. And I like cookies.
6.1
In most cases, a paragraph should express a single thought and the first sentence of a paragraph should articulate what that thought is.
I like my family. I like my brother. He always brings me cookies. And I like cookies. I like my sister as well. She is sweet.
I don’t, however, like capitalism. I mean, people should have rights. Even those of them who don’t have property.
7
Pronouns. Use they when refering to a random or hypothetical person. Not he (sexist), not he or she (needlessly long and distracting) e.g. “The person who cannot bring himself or herself to use “they” needs to get his or her shit together and speak to his or her schoolteacher.”
8
Emphasis. Use it to highlight the word or phrase that conveys the main reason why a given sentence is there, if it is not already obvious, as it is in most cases.
I didn’t have my breakfast today and I was too tired to do my homework.
I didn’t have my breakfast today and I was too tired to do my homework.