Effective writing means you use the language appropriately.
Indoor and Indoors: The former is an adjective, while the latter is an adverb.
e.g. Bowling is an indoor sport.
e.g. It's raining; let's go indoors.
Pretense and Pretension: Pretense (Br. English "pretence") is make-belief; pretension is a claim.
e.g. She made a pretense to faint in front of the audience.
e.g. Your pretension to the money is groundless.
Welcome and Welcomed: The former is an adjective, while the latter is a participle.
e.g. You are most welcome (i.e. you are free) to take whatever you need..
e.g. The Queen was welcomed by the President of the United States
Infer and Imply: Infer means draw a conclusion from; imply means to suggest.
e.g. I can infer from what you said that you don't like him.
e.g. Your comments imply that she was not speaking the truth.
Await and Wait: Await must have an object (meaning be in store for); wait for a person or a thing.
e.g. A big fortune awaits the person with the winning lottery ticket.
e.g. I wait for my wife here.
Forbidding and Foreboding: Forbidding means discouraging; foreboding means suggesting in advance.
e.g. The embassy with its heavy iron gates has a forbidding appearance.
e.g. Look at the dark clouds and high winds foreboding an imminent storm.
Beside and Besides: Beside means next to; besides means in addition to.
e.g. He was sitting beside the President.
e.g. Besides the difficulties, you must also consider the costs of these projects.