How Do You Spell DOST A FAVOR?
Pronunciation: [dˈɒst ɐ fˈe͡ɪvə] (IPA) 
The phrase "dost a favor" is an archaic form of expressing the act of doing a favor. The word "dost" is the second-person singular present tense of "do" in early modern English. It is pronounced as /dʌst/ in IPA phonetic transcription. The spelling with an "o" is due to the Middle English pronunciation of the word. Therefore, the spelling "dost" is no longer used in modern English, and the phrase is substituted with "do a favor" instead.
