Types of "if"

لَوْ

A harf; used when you are looking back at events. It is also used for hypothetical statements. ل indicates the start of the second phrase


Example:

لَوْ أَنزَلْنَا هَذَا الْقُرْآنَ عَلَى جَبَلٍ لَّرَأَيْتَهُ خَاشِعًا مُّتَصَدِّعًا مِّنْ خَشْيَةِ اللَّهِ
Had We sent down this Qur'an on a mountain, verily, thou wouldst have seen it humble itself and cleave asunder for fear of Allah. Al-Hashr [59:21]


إِن

A harf; used for connecting two future events. For instance, 'if something happens, something will happen'. If إِن enter on a ماضى (past) verb, then the meaning of the verb changes to present.

انْ جَاءك زيدٌ
here the verb - جَاء - is a ماضى (past) verb, but the translation is, if Zaid comes to you. The verb has changed to مضارى (present)
If it enters on a مضارى (present) verb, then the meaning is restricted to مستقبل (future)


Example:

وَإِن كُنتُمْ فِي رَيْبٍ مِّمَّا نَزَّلْنَا عَلَى عَبْدِنَا
And if you doubt any part of what We have, bestowed from on high, step by step, upon Our servant [Muhammad]...... 2:23