Step 1: Define both sentence types:
A simple sentence has only one clause. It has one subject and one finite verb and expresses one complete idea.
A complex sentence has one main (independent) clause and at least one subordinate (dependent) clause.
The subordinate clause cannot stand alone as a sentence.
Step 2: Difference 1: number of clauses:
A simple sentence has a single independent clause only.
A complex sentence has an independent clause plus one or more dependent clauses.
Step 3: Difference 2: use of connectors:
A simple sentence needs no subordinating word.
A complex sentence joins its clauses with a subordinating word such as because, although, if, when, who, which or that.
Step 4: Examples:
Simple: The manager signed the letter.
Complex: The manager signed the letter because the client had agreed to the terms.
Here the part after because cannot stand alone, so it is a dependent clause.
Final Answer:
A simple sentence has one clause and no connector. A complex sentence has a main clause plus a subordinate clause joined by a subordinating word.