There are three different ways to end a sentence. There is a correct use for each, despite the fact that they are frequently confused.
The full stop is used at the end of a sentence that is not a question or an exclamation.
The question mark is used at the end of a question. This may seem obvious, but it can be misused. Make sure that the sentence is a question, and always do use a question mark when asking a question – even if it’s a heading or a title.
Can you help me fill out this form? – This is a question, so uses a question mark.
I wonder if you could help me fill out this form. This is not a question, so uses a full stop.
If only part of the sentence is a question, it needs to be split into two sentences
Could you help me with this, because I don’t understand it.
This is a bad sentence – split it into two sentences instead:
Could you help me with this? I don’t understand it.
Exclamation marks are used for an exclamation.
Down with this sort of thing!
Exclamation marks to show excitement
Never use an exclamation mark to denote excitement in a formal setting. The writing should convey this.
Using multiple exclamation marks
You would never use more than one full stop, or more than one question mark. Never use more than one exclamation mark.
Spaces and sentence-ending punctuation
Generally, one space – and no more – should follow a full stop, exclamation mark or question mark. Style guides may differ in this regard. Editors fight mercilessly over it. Whatever you do, be consistent.