Prepositions are probably the most difficult hurdle to overcome, both for the Bulgarians studying English, and for the English struggling with Bulgarian. When it comes to prepositions, almost everyone is baffled at the perceived lack of logic in the target language, while overlooking the exact same lack of logic in the use of prepositions in their native language.

Prepositions, as the Cambridge Dictionary points out, are words that are used before a noun, a noun phrase, or a pronoun, connecting it to another word, and in that they lack any specific meaning of their own, which means that they acquire meaning only in their combinations with other words in the sentence.

For that reason, we cannot assume that prepositions have “translations” in the same sense that other words have, with all the caveats that “translations” entail (e.g. table translated to Bulgarian is (more often than not) маса, and, if we go in the opposite direction, бележник is (very often, though not always) notebook. But the use of prepositions is more or less arbitrary, and even native speakers, when confronted with the question about what exactly a particular preposition means, arrive at the conclusion that there is no clear and definitive answer to that.

What exacerbates the problem is the fact that, at the beginning of every language course, students encounter prepositions in a few restricted use cases (e.g. for spatial relations) and are very easily left with the impression that words like in, on, under, etc. have exact translations in the target language, and later try to use those perceived translations in the subsequent use cases they encounter:
в стаята – in the room
на бюрото – on the desk
at 5 o’clock – в 5 часа
on Monday – в понеделник
depend on – завися от

The trouble with prepositions is that they have no translations, they have equivalents, for example:

at six o’clock – в шест часа
at home – вкъщи
at work – на работа
he gestured at the shelves – той посочи към рафтовете
at a high speed – с висока скорост
at the mention of – при споменаването на
at the door – на вратата (There is someone at the door – Има някой на вратата) BUT: on the door – на вратата (There is something written on the door – Има нещо написано на вратата)

ходя по тревата – walk on the grass
говоря по телефона – speak on the phone
ходя по улицата – walk along the street
пет по пет е двадесет и пет – fives times five is twenty-five
по заповед – by order
по навик – out of habit
по Коледа – at Christmas
един по един – one by one
професор по история – professor in history BUT: учебник по физика – a Physics textbook

Sometimes, one language will not require the use of a preposition, while the other will:
Чакам X влака. – I am waiting FOR the train.
Слушам X музика. – I am listening TO music.
Помагам на Том. – I am helping X Tom.
Свиря на пиано. – I play X the piano.
Том е на 25 години. – Tom is X 25.

Some other prepositions presenting difficulties to either Bulgarians or the English:

Tom is going to the cinema. – Том отива на кино.
Tom is going X home. – Том си отива у дома/вкъщи.

at the moment – в момента
in a moment – след минута

He drinks coffee in the morning – Той пие кафе X сутрин.

before (an event) – преди (събитие)
before the lesson – преди урока
before the war – преди войната
before the movie – преди филма
(some time) agoпреди (отрязък от време)
a week ago преди една седмица
five days ago преди пет дни

I have known John for five years. – Познавам Джон от пет години.
I have known John since January. – Познавам Джон от януари.

What is evident is the complete lack of any logic in the choices the two languages have made about expressing the same relationships when prepositions are concerned. Therefore, one cannot “learn” the rules of using a certain preposition; the best course of action is to simply memorize the combinations, one (or a few) at a time, until the relevant phrases become internalized and their usage becomes automatic (mostly through repetition).

What I require my students to do to achieve this, is to make a note of every preposition they encounter, and, after asking themselves the question: “Can I produce this exact phrase in the target language if I need to?”, to write the whole sentence down and make sure they circle the preposition in the cases when the answer is negative or when they are not certain about it. Usually, in a couple of months, they become more confident and a feeling emerges that, even if the war still seems impossible to win, they at least have managed to triumph in a few battles.