He had to forswear on the stand because they threatened his family.
I tried to forswear eating desserts but I could not do it.
He promised to forswear smoking but he still would when out with his buddies.

On our wedding day, I promise I will forswear my love for you at the altar.

I vow to forswear all lies and deceit in my pursuit of honesty and integrity.

The word foreswore does not seem to exist. The word is forswore which is the past participle of forswear meaning to "make a serious decision to stop doing something"

"Did my heart love 'till now? Forswear it sight, for I ne'er saw true beauty 'till this night."

He was overwhelmed with desire, passion and admiration for Juliet, and realized that his feelings for Rosaline were shallow and trivial.