4 ideas
A poet should not describe an object from outside through their own preconceptions. They should drop self-intent and enter the object until its hidden inner quality becomes visible, so that feeling arises from the thing itself. A verse built from the poet's cleverness rather than this entry into the object is artificial, however skillful it looks.
Haikai has two aspects. One is the permanent (fueki), the lasting truth that makes a verse endure. The other is the fleeting (ryūkō), constant novelty that keeps the art alive. Both come from the same sincerity (makoto), so a poet who chases only novelty becomes shallow and one who clings only to tradition goes stale.
He chose 'furuike ya' (old pond) and paired it with the plain sound of a frog jumping into water. Breaking the inherited association and grounding the verse in a direct, overlooked perception is what made haikai a new kind of poetry.
Bashō distinguished verses that 'become' (naru), arising when the poet's mind merges with the object, from verses that are 'made' (suru), assembled by deliberate effort when no such merging has happened. A 'made' verse shows the poet's intent and lacks life. The remedy is to write swiftly while the perception is still hot, rather than polishing an idea after the moment has cooled.