Hi, Judit
It's totally fine to ask any questions
Even if something was already asked on forum before, then no guarantees that the situation didn't change with time, better to reask and be sure about current day information
Regarding your question about Budapest to Bratislava, then the answer is really simple - in relation between Hungary and Slovakia return tickets cost less than one-way (sounds weird, but such things sometimes happen in Central Europe) and in the light of these Hungarian Railways (MAV) doesn't sell one-way tickets to Slovakia, but on the contrary sells return one (=cheaper). So if you search for return ticket, then you'll see something like
And can easily buy this ticket any moment, the return part of the ticket just won't be used by you
However, do you really need to buy Budapest-Bratislava ticket in advance? The point is that none of the trains on this route required seat reservation, thus can't sold out, and the price of the ticket is always the same no matter when you buy your ticket (it's not like in Germany when in advance purchase can get you much lower price), as result most of the passengers just buy tickets like Budapest to Bratislava before departure and don't bother with not very friendly MAV eShop interface & further printing of the ticket via MAV ticket machine