Okay this is quite a tricky one. what's the past tense of welcome?

welcomed

explanation:

present tense  

he/she/it   -   welcomes

present participle -   welcoming

past tense - welcomed

past participle - welcomed

first of paragraph? and second of statment?

don't submit such stupid questions!

type of error:

the error in the sentence is a "dangling modifier". a dangling modifier is a phrase, word or clause that "dangles" because there is not target to modify. in this case, "after mopping the floor" is the dangling modifier since it has no word to modify. the noun "house" cannot mop the floor; therefore there is a mistake in the writing of the sentence.