Compound sentences consist of two independent clauses joined together using a coordinator, a conjunctive adverb, or a semicolon.

Coordinators

Coordinators are seven words (also called coordinating conjunctions) that make the popular phrase FAN BOYS. We usually use a comma after the first independent clause and before the coordinator.

• For (to add a reason) 

Smartphones cause serious psychological issues, for they have addictive features.

• And (to add a similar, equal idea)

Smartphones cause serious psychological issues, and they can affect one’s physical health.

• Nor (to add a negative equal idea) (question word order)

Smartphones should not be used for more than two hours a day, nor should they be carried by children.

• But (to add an opposite idea)

Smartphones should not be carried by children under the age of five, but they can be helpful in cases of emergencies.

• Yet (to add an unexpected or surprising continuation)

Smartphones are designed to be addictive, yet they are of good use.

N.B.: Yet and but have similar meanings with only one difference that is whether the continuation of the sentence is expected or not. 

• Or (to add an alternative possibility)

Parents should monitor their kids’ screen time, or they risk the children growing too attached to their smartphones.

• So (to add an expected result)

Smartphones are deliberately designed to be addictive, so many young adults suffer from anxiety when they limit their smartphones’ screen time.

N.B.: And, but, and or are used in simple sentences to create a compound verb or compound subject.

Conjunctive adverb 

Usually punctuated by a semicolon before the conjunctive adverb and a comma afterward.

• To add a similar, equal idea, we use the following words: also, besides, furthermore, in addition, moreover, as well, and too. (an equivalent of and)

Smartphones cause serious psychological issues; besides, they can affect one’s physical health.

Smartphones cause serious psychological issues; they can affect one’s physical health as well.

Smartphones cause serious psychological issues; they can affect one’s physical health, too.

N.B.: As well and too are identical, punctuated differently though, as there is a comma before too. 

• To add an expected or surprising continuation, we use the following words: however, nevertheless, nonetheless, and still. (an equivalent of yet)

N.B.: Still is preferred while speaking not in writing.

Smartphones are designed to be addictive; nevertheless, they are of good use.

• To add complete contrast, we use the following phrases: on the other hand, in contrast. (This could be punctuated with a semicolon before the phrase and a comma afterward; however, it is preferred to be punctuated as two separate simple sentences with a period to end the previous sentence and a comma following the phrase.)

Smartphones are designed to be addictive. On the other hand, they are of good use.

• To add an alternative, we use the following words and phrases: as a result, consequently, hence, therefore, and thus. (an equivalent of so)

Smartphones are deliberately designed to be addictive; as a result, many young adults suffer from anxiety when they limit their smartphones’ screen time.

• To add an example, we use the following phrases: for example, and for instance.

For example, a teenage student at a high school may need to follow up with the latest news and rumors that run in their school, so they can keep up with their social life.

Semicolons

We can combine two independent clauses that are closely related to each other using a semicolon. 

N.B.: Do not overuse it. It could be used only one time. Many modern writers tend not to use it at all.

Smartphones cause serious psychological issues; they can affect one’s physical health.