Passage
nor by thy head mayest thou swear, because thou art not able one hair to make white or black;
nor by thy head mayest thou swear, because thou art not able one hair to make white or black;
Matthew 5:34 but I--I say to you, not to swear at all; neither by the heaven, because it is the throne of God,
Matthew 5:35 nor by the earth, because it is His footstool, nor by Jerusalem, because it is a city of a great king,
Matthew 5:36 nor by thy head mayest thou swear, because thou art not able one hair to make white or black;
Matthew 5:37 but let your word be, Yes, Yes, No, No, and that which is more than these is of the evil.
Matthew 5:38 `Ye heard that it was said: Eye for eye, and tooth for tooth;
The verse centers on "head", "mayest", "thou", "swear", "able", "hair", and "make". It is saying that the verse draws attention to "head" and "mayest", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 35's "nor by the earth because it is..." into verse 37's "but let your word be Yes Yes...", so "head" and "mayest" belong inside that flow. In Matthew context, the local focus is Christ, faith, and discipleship.
A plain takeaway is to answer the verse's own emphasis on "head" and "mayest" with trust shaped by these words, not by a vague optimism outside the passage.