To is a versatile preposition. A few of its many definitions are (1) toward, (2) reaching as far as, and (3) until.1 Too is an adverb meaning (1) additionally, (2) excessively, (3) very, or (4) extremely.2 Whenever you’re in doubt about whether to use to or too, see if any of those synonyms of too (i.e., additionally, extremely, etc.) would work in its place. If none fits, then to is probably the word you’re looking for.
Usually when someone uses to in place of too or vice-versa, it is simply a typo or an error made in a careless moment, so let’s not be too hard on people who occasionally mix them up. None of us is immune to such mistakes. When the mixup is habitual, however, it is a problem. Most people who speak English as a first language master the distinction in primary school, so the mixup can make one look either poorly educated or like a very unpracticed writer, which can be devastating when it comes to college applications or job-application cover letters.
I am going to bed.
She turned to him and said hello.
The dictator was restored to power.
He pressed his face to the glass.
We stood face to face.
There were two men to every woman.
It’s now ten minutes to six.
I came to return this book.
When I came to, I had blood on my hand and everyone was looking at me.
I pushed the door to and shut off the lights.
You can’t have your cake and eat it too.
The sun was too bright, so I put on my shades.
The error was too glaring to ignore.
She wasn’t too pleased to see us again.
You will too clean your room!
I miss you, too.