The most common grammatical error I see on Figment is to do with dialogue tags. Usually it will go something like this:
"I'm going to get a new apple before I pay." She said.
The problem with this is that it is two sentences when it should be one. 'She said' is not a sentence. It is 'tagging along' with the previous sentence in order to explain it. This is called a dialogue tag. The correct punctuation for the sentence above is:
"I'm going to get a new apple before I pay," she said.
Notice that the period turns into a comma and the 'she' is not capitalized. This is because 'she' is not the beginning of a new sentence.
But what if there is a question mark or an exclamation mark, or some other kind of punctuation at the end of the dialogue? In this case, skip the comma but keep the tag in lower case:
"May I get a new apple before I pay?" she asked.
"I'm getting a new apple before I pay whether you like it or not!" she yelled in the cashier's face.
"I guess I'll get a new apple before I pay..." she muttered.
Of course, if the subject of the tag is a name, then leave the name capitalized:
"I'm going to get a new apple before I pay," Sally said.
Dialogue tags can also be found before the dialogue, in which case the period at the end of the tag is replaced with a comma instead of the period at the end of the dialogue:
Sally said, "I'm going to get a new apple before I pay."
Also, the tag could be surrounding the dialogue in which case commas would on either side like so:
Sally said, "I'm going to get a new apple before I pay," before walking to the produce aisle.
So when don't you use the comma? Well, sometimes there isn't a dialogue tag in the first place. In the examples above, the main thing to remember was that they were all the same sentence. Sometimes instead of a dialogue tag there is a separate sentence that may explain the dialogue indirectly:
"I'm going to get a new apple before I pay." She left the rest of the apples at the cash register and walked to the produce aisle.
We assume that 'she' was the one who said the previous dialogue, but the dialogue is separate from the second sentence. Notice that in this case there is a period and the next sentence is capitalized. This is because the next sentence is a sentence of its own and not a dialogue tag.
The last thing I can think to explain is the use of dashes in dialogue. They can be used to show that a character stops talking in the middle of a sentence or they can be used to put action in the middle of dialogue:
"I'm going to get a new apple bef—"
Sally was cut off by the cashier throwing the bruised apple at her face.
"This apple is bruised—" Sally motioned towards the produce aisle. "—so I think I'm going to get a new one before I pay."
Notice that in the second example the second half of the dialogue is not capitalized. This is because it is a continuation of the dialogue from before.
On a side note, you may be wondering whether you should use single quotation marks ( ' ' ) or double quotation marks ( " " ). Well, both are correct, it just depends on the country living in. Personally, I use doubles because I'm from Canada, but either one is correct. If your character is describing dialogue within dialogue, use the other type to avoid confusion:
"So I said, 'I don't want this apple,' and she totally freaked out!" Sally exclaimed as her mother snoozed on the other side of the phone.
'So I said, "I don't want this apple," and she totally freaked out!' Sally exclaimed as her mother snoozed on the other side of the phone.
I hope this helped clear the waters about dialogue! Happy writing!