は
tooth
ha
JMdict 1313000
はを歯をtoothみがこう磨こうlet's brush
を (object) marks 歯は (teeth) as the thing being acted on. The verb 磨こうみがこう (let's brush) is the volitional form of 磨くみがく (to brush) — the "let's…" / "I'll…" shape, casual and self-encouraging rather than a command.