(Naseba naru; “If you do it, it will happen.”)
A person can do anything if they really put their mind to it. Where there’s a will, there’s a way. The only way to get a thing done is to do it.
We begin with the verb 為す (nasu), “to attempt,” “to do,” “to accomplish,” in perfective form and taking the conditional suffix ば (ba); this is followed by the verb 成る (naru), “to become,” “to result in,” in sentence-final form.
An extended version adds the inverse and a further comment, ～為さねば成らぬ何事も. This also begins with 為す, except in imperfective form. This is followed by the negative suffix ず (zu) perfective form as ね (ne), followed by the conditional ば. Similarly, the second 成る appears in imperfective form and takes the negative ず, which in turn appears in the prenominal form ぬ (nu). This allows it to precede the noun 何事 (nanigoto), “something.” And finally we get the emphatic particle も (mo), effectively changing the preceding “something” to “anything.”
The longer form, then, reads as “If you do it, it will happen; if you don’t, it won’t, and thus for all things.”
The longer form of this phrase is taken verbatim from a poem by Uesugi Youzan (aka Uesugi Harunori), an Edo-period daimyo famed for returning a deeply-indebted domain to prosperity through a combination of financial discipline (for example, by cutting his own household budget by 86%!) and support for education and industry. The poem continues, but the two versions presented above are what appear in kotowaza dictionaries, and the shorter version is the more common of the two.
However, note that the basic 為せば成る idea predates Uesugi, dating at least to a similar poem by Takeda Shingen that Uesugi seems to have been playing on.
Some people replace 為す with homophone and near-synonym 成す. This is not considered strictly incorrect. However, the nuance of the former is “to do,” while the nuance of the latter is “to accomplish.” So replacement phrase 成せば成る becomes a mere tautology, “If you get it done, you get it done.” The original version is preferred because it retains the message of active effort leading to results.
(Youta wa kouen ni tsuku to mohaya ikigire de kurushii omoi wo shiteita kedo, jumon no you ni, naseba naru, naseba naru to tsubuyaki nagara sanshuu hashitte mawaru to sukkiri shita.)
[Youta was already struggling and out of breath when he reached the park. But he managed to run three laps, murmuring If you do it, it gets done; if you do it, it gets done to himself like an invocation, and felt much better afterwards.