In English we make a difference between a specific thing and a general thing. "the" means the thing is specific - we have just talked about it, or it is unique or there may be many but we are interested in this particular one. "a" means the thing is general - I have never seen it before, or it is just one in a group, there may be many and it doesn't matter which one. Russian works fine without "a" and "the". It is just a necessary part of grammar in English.