1)There were tens of students. Yes it makes sense but you don't often hear 'tens' used in the same way as 'hundreds' or 'thousands'. It's a little strange. More commonly someone would estimate how many people there were to the nearest multiple of 10 and write something like 'There were around fifty students'. Either that or say 'dozens' instead which is more commonly used. 2) Almost grammatically correct. In your example it could be 'Meeting foreigners widens your horizons' or 'Meeting foreigners will widen your horizons'. 'Widen your horizons' means you open your mind to other parts of life, and different possibilities, which meeting people from different countries can definitely do so yes it makes sense. 3) The thing you are trying to express makes sense but the construction 'self-motivate with' sounds very odd to me. I would write it either as: Since you do not have a fixed schedule you must (be able to) self-motivate, and do your homework in order not to fail the course. or Since you do not have a fixed schedule you must motivate yourself to do your homework in order not to fail the course.

Instead of "tens," try using "dozens." There were dozens of students. 1 dozen = 12 dozens = 12 many times over Meeting foreigners widenS your horizons - enrichES you - openS your mind Remember to put an "s" on the end of these verbs. 'Meeting foreigners' is the subject and equal to the pronoun 'it.' When you're using 'it,' your verbs adopt an s. The last sentence is missing a couple of commas, but otherwise it looks good to me!