Its and it’s
It’s — always short for “it is.”
Its — always means “of it” or “belonging to it.”
It’s hot today. My elephant hates its saddle.
Breath(e)
When you take a breath, you breathe.
Breath is a noun, breathe is a verb.
Do not say, “I like to breath fresh air.”
Rein and reign
You control a horse with reins. You can rein it in, or “give it the reins” to let it run.
A king or queen reigns; his or her time as monarch is the reign.