Bromo33333

I expect to pass through life but once. If therefore, there be any kindness I can show, or any good thing I can do to any fellow being, let me do it now, and not defer or neglect it, as I shall not pass this way again.

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“Speak properly, and in as few words as you can, but always plainly; for the end, speech conveys not ostentation, but understanding.” - William Penn (trans to e-prime)
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Bromo33333

ZeroCapacity

I expect to pass through life but once. If therefore I can show any kindness, or do any good deed for anyone else, let me do it now, and not defer or neglect it, as I shall not pass this way again.

ksaupe

I expect to pass through life but once. If therefore, there be any kindness I can show, or any good thing I can do to any fellow being, let me do it now, and not defer or neglect it, as I shall not pass this way again.

How about: If therefore, I can show any kindness, or do some good to any fellow human, let me do it now....

or, more clunkily:

If therefore some opportunity arises to show a kindness or do some good to any fellow human, let me do it now....

Weetabix

I like ZeroCapacity's translation. I might alternatively substitute "creature" for "being" to stay more with the style or flavor of William Penn's time.