Learn Arabic (Modern Standard): نظف الجرح بهذا المطهر.

This is because نظف is a command (imperative) addressed to a single male. In Arabic, regular commands given to a masculine 'you' typically end with a sukoon (a marker showing the absence of a vowel). If you were telling a woman to clean the wound, you would add an -ee sound at the end: نظفي.

Why are the words for 'with' and 'this' attached in بهذا?

The word بهذا combines the preposition بـ (meaning 'with' or 'by means of' for tools/instruments) and the demonstrative pronoun هذا (meaning 'this'). Single-letter prepositions in Arabic, like بـ, لـ, and كـ, never stand alone and always attach directly to the front of the word that follows them.

Why does المطهر have the definite article الـ (al-) attached to it?

In Arabic, when you want a phrase to mean 'this [noun]', the noun following the 'this' must be definite, meaning it takes الـ. Because we are saying 'this antiseptic', we say هذا المطهر. If you dropped the الـ and just wrote هذا مطهر, the grammar changes completely and it becomes a full sentence meaning 'This is an antiseptic.'

Does the word for antiseptic, المطهر, share a pattern with other Arabic words?

Yes! It comes from the root letters for purity (ط - ه - ر). It is built using a very common pattern for the 'doer' of an action (an active participle) from a Form II verb. The مـ (mu-) prefix combined with an 'i' sound before the last letter shows that it is the thing performing the action. It literally means 'that which purifies or disinfects.'