I ate a cheeseburger and fries.

That is a much more concise, clean sentence.

I understand that your teacher probably wanted you to just plunk semi colons into those sentences, but why do that when there are MUCH better ways to write them? Blow her mind and show her you know not only good grammar, but good, concise writing.

I went to Walmart because we needed food for the weekend.

I am going to the mall, but I might go skating instead.

see what I mean?

It binds two sentences more closely than they would be if separated by a full stop/period. It often replaces a conjunction such as and or but. Writers might consider this appropriate where they are trying to indicate a close relationship between two sentences, or a ‘run-on’ in meaning from one to the next; they might not want the connection to be broken by the abrupt use of a full stop.

It is used as a stronger division than a comma, to make meaning clear in a sentence where commas are being used for other purposes. A common example of this use is to separate the items of a list when some of the items themselves contain commas.

There are several cases where semicolons may be used:

Use a semicolon between closely related independent clauses not joined by a coordinating conjunction: “I went to the pool; I was informed that it was closed.”

Use a semicolon between independent clauses linked with a transitional phrase or conjunctive adverb: “I like to eat cows; however, they don’t like to be eaten by me.”

Use a semicolon between items in a series containing internal punctuation: “There are several Waffle Houses in Atlanta, Georgia; Greenville, South Carolina; Gainesville, Florida; and Mobile, Alabama.”

A semicolon can be used to separate independent clauses that are joined by coordinating conjunctions when the clauses have internal commas that might lead to misreading: “After the game, I won a red beanie baby, four edible ingots, and a certificate of excellence; but when the storm came, I lost it all in a torrent of sleet, snow, and profanity.”

Semicolons are always followed by a lower case letter, unless that letter begins a proper noun. Semicolons are placed after closing quotation marks.