Apart and A Part are easily confused terms. One is a single word, the other is a noun phrase.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary, but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted, or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Apart (pronounced “uh-pahrt”) is an adverb. It describes the state of being in pieces. For example if you have a toy kit, it comes with its pieces all apart. You put them all together to complete the toy, then take it apart again to pack it back in the box.
In a figurative sense, apart can describe a lost, hurt emotional state. For example, “Every now and then I fall apart” is a line from the 1980s track “Total Eclipse of the Heart” (sung by Bonnie Tyler, written by Jim Steinman.)
A Part (pronounced “uh” (pause) “pahrt”) is a noun phrase. It describes one piece or component in a set. For example, regular refrain of US TV 1980s cereal ads was “____ cereal was a part of a healthy breakfast.” And the ad showed cereal, eggs, and bacon.
The following story uses both words correctly:
After the kids and his wife had gone to bed, Apsel eagerly assembled his son’s Lego kit. He felt seven years old all over again. He was just about done building the model plane when he realized he was missing a part or two.
He looked around, then under a nearby couch. There they were. Apparently when his son had been playing with the model, some pieces had come apart from it, and he hadn’t noticed.
He added the last two pieces to the model. Ah, finished. Kids today have it so good, he thought.