Learn Hardcore Japanese: kokuban ga yogoretara, kesigomu de keseba kirei ni naru. - If the blackboard gets dirty, erasing it with an eraser makes it clean.

が marks the subject that undergoes the action or change—in this case, the blackboard getting dirty. It presents 黒板 as new or focal information. If you used は, you’d be topicalizing the blackboard (“as for the blackboard…”), which can imply contrast or that the board is already an established topic. Here, が is more neutral for stating the condition.

Why do we use 汚れる instead of 汚す in 汚れたら?

汚れる is an intransitive verb meaning “to become/get dirty,” whereas 汚す is its transitive counterpart meaning “to dirty (something).” We want to express “when the board gets dirty” (it’s in that state), so we use the intransitive 汚れる. If you said 汚したら, it would mean “when someone dirties the board,” changing the nuance.

What does the -たら form (汚れたら) indicate here?

The -たら form expresses a conditional or temporal clause: “when/if.” In 汚れたら, it means “when (or if) the blackboard gets dirty.” It can describe a single future event or a habitual/general truth. Here it sets up a rule: whenever the board is dirty, do the next step.

Why is there で after 消しゴム in 消しゴムで消せば?

The particle で marks 消しゴム as the instrument or means by which the action is done. So 消しゴムで消す literally means “erase with an eraser.”

What is the function of 消せば, and how does the -ば conditional differ from -たら?

消せば is the -ば conditional form of 消す, meaning “if (you) erase.” Differences among major conditionals: • ば: straightforward “if,” good for general/hypothetical conditions.
• たら: “when/if,” more temporal and colloquial.
• と: implies an automatic or inevitable result.
In this sentence, ば smoothly links the general condition “if you erase it” to the result “it will become clean.” Using と would sound too mechanical, and たら was already used in the first clause.

Why do we say きれいに instead of きれい in きれいになる?

きれい is a na-adjective. To use it adverbially or before なる (“to become”), you add に: きれいに. The pattern [na-adjective] + に + なる means “to become [adjective],” so きれいに＋なる = “become clean.”

Why do we use なる instead of する (i.e., きれいになる vs. きれいにする)?

なる expresses that the subject changes state on its own or by an implied cause (“the board becomes clean”). する would mean “to make [something] clean,” emphasizing the agent’s action. Here the focus is on the board’s state change after erasing, so きれいになる is more natural.

Why is there no explicit subject in 消せば and きれいになる? Who is doing the erasing?

Japanese often omits subjects when they’re clear from context. In the first clause, 黒板 is the subject. In 消せば, the implied subject is “you” (or “one”) performing the erasing. In きれいになる, the subject reverts to 黒板 as it undergoes the change. Repeating them isn’t wrong but is usually unnecessary in Japanese.