Recommended by Isabela
"THEN" and "THAN"
"Then" relates to time. "Than" is used for comparison. Ex.: First, we'll have lunch, and then we'll go for a walk. He is more experienced than I am.
"WEATHER" and "WHETHER"
"Weather" refers to the atmospheric conditions. "Whether" is used to introduce alternatives or choices. Ex.: The weather is perfect for a picnic. I'm not sure whether to wear a jacket or not.