Hebrews 6:4-8
For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
This passage does not teach that one can lose his salvation through disbelief or apostasy. These verses refer to a hypothetical situation wherby the author stresses what would happen to a saved person if he could fall away. The author does not believe one can lose his salvation, or that his readers had{verse 9}; but he so speaks to demonstrate the folly some might have in imagining that they can turn back to Judaism without suffering loss.
Though the author is not writing about his readers{verse 4 those}, he still is writing for their sakes{verse 9,you }. If they shall fall away is the translation given to the fifth participle of the passage. This is a legitimate and even common grammatical usage by the author within the warning passages{2:3; 10:26}and even 6:8.
The first four participles refer to actual blessings whereas number five describes potential situation, and so can be translated unlike the first four.
God Bless!