Grammar JLPT Level N4 - Lesson 23

In the same way as the case marking particleと means 'and', sometimes it is translated as 'if'. In these cases, it is considered to be far stronger than ば, なら, or たら (which express hypothetical outcomes). This is because one of the primary uses of と is highlighting a result. In these cases, と is actually a conjunction particle, and shows one of three things.
仮定(かてい)の順接(じゅんせつ) - A result that is assumed to be true, and obvious.
確定(かくてい)の順接(じゅんせつ) - A result that is known to be true.
仮定(かてい)の逆接(ぎゃくせつ) - The opposite of a result that is assumed to be true.
Due to this, the (B) part of a phrase using と as 'if', will always occur when (A) happens, as it is considered to be the result of (A).

• 甘(あま)いものをいっぱい食(た)べると、太(ふと)る。
  If you eat a lot of sweet things, you will gain weight.
• 部屋(へや)が汚(きたな)いと、お母(かあ)さんに怒(おこ)られる。
  If my room is messy, my mother will yell at me.
• 部屋(へや)が静(しず)かだと、眠(ねむ)れない。
  If my room is quiet, I can't sleep.
• 地下鉄(ちかてつ)だと、5分(ふん)早(はや)く着(つ)く。
  If it's the subway, you will arrive 5 minutes faster.

と may not be used in situations where the (B) result 'might' happen. It must be definite (or close to definite).

• 走(はし)るとけがをする。
  If I run, I will injure myself. (Unnatural Japanese, unless this person always injures themselves while running for some reason)

In this particular nuance of と, despite being translated as 'if', the actual meaning is much closer to 'and', as と is expressing that (A) and (B) will always go together.

• 目(め)を閉(と)じると、何(なに)も見(み)えない。
  I close my eyes, and I can't see.