The words ЕСЛИ, ЛИ, ИЛИ

Fairly often some nuances connected with the structure of the Russian sentences, which are similar to English ones with the word IF, escape the attention of English-speaking learners.
The first and most significant nuance is that the word IF is not always translated into Russian as ЕСЛИ [YESLI]. If you always translate it like this, the prepositional logic will be destroyed.

In the sentences called indirect questions it should be only translated as the word ЛИ [LI].

I don't know IF he comes. = Я не знаю, придёт ЛИ он.

The word ЛИ is not a short form of the word ЕСЛИ [YESLI]. Pay your attention to the position of the particle ЛИ, which always differs from the position of the word IF in the English sentence of this type. It can never be situated at the beginning of the subordinate clause in the Russian language. Moreover, its position is not constant.

He can’t say if they buy the car.

In Russian it can be:

1. Он не может сказать, купят ли они машину.
2. Он не может сказать, машину ли они купят.
3. Он не может сказать, они ли купят машину.

The meaning of these sentences is quite different.

1. Он не может сказать, КУПЯТ ЛИ они машину (= купят ИЛИ нет). - He can’t say if they BUY (or NOT buy) the car.
2. Он не может сказать, МАШИНУ ЛИ они купят (= машину ИЛИ что-то другое). - He can’t say if they buy the CAR (or something different).
3. Он не может сказать, ОНИ ЛИ (= они ИЛИ кто-то другой) купят машину. - He can’t say if THEY (or somebody else) buy the car.

Thus, the particle ЛИ [LI] is a modified form of the word ИЛИ [ILI]. In the Russian sentence the difference in the content of the sentences of this type is determined not only by intonation (like in English sentence), but also by the position of the particle ЛИ [LI]. This particle directly goes after the word which expresses a problem and implies another option:

купит ЛИ (= купит ИЛИ нет / will buy or not);
машину ЛИ (= машину ИЛИ что-то другое / a car or something else);
они ЛИ (= они ИЛИ кто-то другой / they or somebody else).

We meet the second discrepancy of English and Russian structures in conditional sentences, where the word IF is actually translated as ЕСЛИ [YESLI], e.g:

Если ты можешь, приходи пораньше. = If you can (have opportunity), come earlier.

However, you should pay attention that this word in the Russian sentences does not prevent us from using future tense forms.

Compare:
I will help (future tense) her only if she asks (present tense) me.
But in Russian it would be: Я помогу (future simple) ей, только если она попросит (future simple) меня.

Finally, pay attention to the fact that the comma is necessarily written between simple sentences of such structures in the Russian language regardless of which of its clause, superordinate or subordinate, stands first.

In the English version the comma is not written in the case when superordinate clause without the word IF stands first:
We will go (future tense) there every Monday and Thursday if they allow (present tense) us frequent visits.

In the Russian version it would be written anyway:

1. Мы будем ходить (future compound) туда каждый понедельник и четверг, если они позволят (future simple) нам частые визиты.
2. Если они позволят нам частые визиты, будем ходить (future compound) туда каждый понедельник и четверг.