Semicolons scare a lot of people, but they don't need to.
Semicolons:
-separate independent clauses
-are used in place of a period
-are not used with a conjunction (and, but, or, etc.)
-can be used with a conjunctive adverb (however, moreover, therefore, etc.)
Ex: She wanted to go to the store; however, it was closed.
-Use a semicolon when you have two independent clauses that you feel should be linked.
Ex: She spit out her tea; there was a bug in the cup.
-You can format those as two sentences (She spit out her tea. There was a bug in the cup), but it makes sense to combine them because one clause is the reason for the other.
-You could also combine these with a conjunction. Ex. She spit out her tea because there was a bug in the cup.
-It comes down to a stylistic choice.
As a general test, if everything before your semicolon can be its own sentence and everything after your semicolon can be its own sentence, you used it correctly. If not, you need a comma.
Commas:
-signify a pause
-link phrases and clauses together
-separate items in a list (A duck, a swan, and a turtle were in the pond.)
The third one is called an oxford comma. Its necessity is up for debate, but I think you need it.
-come after sentence adverbs (Finally, the movie started)
-are used to format dialogue, as shown here
-come between adjectives modifying a noun (I saw a large, green rock).
As a general test, use a comma if you would naturally pause in the sentence. Try the sentence without one if you're unsure. But, many people overuse commas, which makes the sentence cluttered and confusing. If in doubt, don't use one.
Exercise 1:
Which of the following is the correct use of a semicolon?
1. I saw him the other day; and I stopped to talk.
2. The duck was angry; the swans stole its bread.
3. He rode his bike to the store yesterday; to get a gallon of milk.
Exercise 2:
Where should you put a comma in the following sentences?
1. I saw a duck and a swan fighting over a loaf of bread.
2.I need milk eggs and butter from the store.
3. After an hour we were able to leave.
4. There was a small orange leaf on the kitchen table.
5. "I don't know how to whistle" she muttered.
Answers
Exercise 1:
1. Incorrect. Should be: I saw him the other day and I stopped to talk. (a comma after day could technically be correct, but unnecessary.)
2. Correct
3. Incorrect. Should be: He rode his bike to the store yesterday to get a gallon of milk.
Exercise 2:
1. No commas necessary
2. after milk and eggs. I need milk, eggs, and butter from the store.
3. after hour. After an hour, we were able to leave.
4. after small. There was a small, orange leaf on the kitchen table.
5. after whistle. "I don't know how to whistle," she muttered.