“they” to refer to a company. Use the pronouns it or its. Example: Apple has made many successful products; many people have bought its iPhone and iPad.
“that” to refer to a person or group of people. Examples: Bob is the one who mowed my lawn. I thanked the people who took out my trash. Sally is the player whom they carried off the field after she scored the winning goal.
the plural pronoun “they” to avoid using he/she or his/her. Instead, make the noun plural. A reporter is busy, so you should respect his/her time. Reporters are busy, so you should respect their time.
independent clauses with a word such as however, moreover, therefore, consequently, otherwise, nevertheless, thus I built this online lesson; consequently, you now have some valuable grammar tips.
which, from Grammar Girl • i.e. vs. e.g., from The Oatmeal • Who vs. whom, from Grammar Girl • What it means when you use “literally,” from The Oatmeal • Affect vs. effect, from the Grammarist