There are 3 times you uses a semicolon:
1) to separate 2 independent clauses (could be their own sentence) that are not joined by a conjunction (for, and, nor, but, or, yet, so).
Example: Keith is captain of the squad; Eric is manager.
2) before an adverb (therefore, however, so, consequently) that joins 2 independent clauses.
Example: Lisa is allergic to milk; consequently, she has to follow a special diet.
3) to separate items in a series that are already separated by commas, but need further clarification.
Example: People came from Berkeley, California; Eugene, Oregon; Houston, Texas; and San Diego, California.
In the following sentences, insert semicolons wherever necessary.
1. The contestants came from Dubuque, Iowa Memphis, Tennessee and Pittsburgh, Pennsylvania.
2. I got all dressed up for the party however, my best friend wore jeans.
3. Our school is overcrowded and consequently, a new wing will be added.
4. My father is an accountant my mother is a lawyer.
5. Berman is a difficult language to mater however, if you plan to be a scientist, you may want to study this language.
6. The roof was leaking, and the walls were sagging therefore, the Smiths hired a contractor to repair the house.
7. I've seen photos of Paris, France London, England and Rome, Italy.
8. This case bears investigation I'll put someone on it.
9. Anne is going to Japan this summer therefore, she is studying Japanese now.
10. On our vacation, we drove through San Francisco, California Salt Lake City, Utah and Jackson, Wyoming.
11. The clock on the stairs keeps perfect time its precision is remarkable.
12. Juan is an excellent math student however, he is planning to be a writer.
13. Applies are usually sold by the pound oranges are sold by the dozen.
14. Separate the strands of yarn then tie every other one.
15. She came late to class it was hard for her to catch up.