It's very close. But, you shouldn't follow "one" with "he or she." Try: If one doesn't know English, one definitely falls behind with many things and loses a lot of opportunities. I also changed the end of the sentence because "one" tends to be more formal and "lots" tends to be more casual. It is also somewhat rare for people to use "one." It isn't wrong, it just sets a formal, distancing tone. You could also try: Learning English is a must these days. If a person doesn't know English, he or she definitely falls behind with many things and loses lots of opportunities. Which is a slightly less formal (but still professional) register.

Perfect; only 'fall behind' sounds slightly awkward here. Perhaps 'misses many things' - such as the latest music and films.

Hi Wade! Thank you so much for your very good comment! ;)

I would say "These days learning English is a must. If you don't know English, you miss out on many things and have a lot fewer opportunities." You don't have to change the order of the first sentence, but I think it sounds slightly better. You wouldn't say "lose" for opportunities. It's a missed opportunity. However, to avoid repetition, I would rephrase it this way. Hope it's helpful.