לפני ששמתי פלסטר חדש, חיטאתי שוב את הפצע כדי שהוא יישאר נקי.

לפני means before. When Hebrew follows it with a full clause, it usually adds ש־:

• לפני ששמתי... = before I put...

So ששמתי is just:

• ש־ = the connector
• שמתי = I put / I placed

The double ש is completely normal: one belongs to the connector, and one belongs to the verb.

שמתי is the past-tense I form of לשים or לשים/לשים? In standard modern usage here, it means I put / I placed.

Hebrew uses the past because the speaker is telling a past story: both actions already happened. So even though the disinfecting happened before the bandage was put on, the act of putting on the bandage is still a completed past event from the speaker’s point of view.

A more general statement would use present or another form, for example:

• לפני ששמים פלסטר חדש... = before putting on a new bandage... / before one puts on a new bandage...

פלסטר means adhesive bandage, often like Band-Aid in everyday English.

It is a very common everyday word in Hebrew. Grammatically, it is masculine singular, which is why the adjective after it is חדש and not חדשה.

In Hebrew, adjectives usually come after the noun they describe.

So:

• פלסטר חדש = a new bandage
• הפלסטר החדש = the new bandage

This is one of the big word-order differences from English, where adjectives usually come before the noun.

חיטאתי is the past-tense I form of לחטא, which means to disinfect or to sanitize.

The ending ־תי is a very common past-tense ending meaning I:

• שמתי = I put
• חיטאתי = I disinfected

So this sentence has two first-person past verbs: שמתי and חיטאתי.

Hebrew uses את before a definite direct object.

Here, הפצע means the wound, and it is definite because of ה־. So Hebrew marks it with את:

• חיטאתי את הפצע = I disinfected the wound

If the object were indefinite, את would usually not appear:

• חיטאתי פצע = I disinfected a wound
  though in real life this wording is less likely in this context

שוב means again.

So:

• חיטאתי שוב את הפצע = I disinfected the wound again

Its position is fairly natural here because it sits close to the verb. Hebrew does allow some flexibility, and you may also hear things like:

• שוב חיטאתי את הפצע
• חיטאתי את הפצע שוב

But the sentence as given is very normal.

כדי ש means so that or in order that. It introduces a purpose clause.

So:

• כדי שהוא יישאר נקי = so that it will stay clean / so that it stays clean

Hebrew commonly uses a future form after כדי ש because the speaker is expressing an intended result. Even though the main verb is past, the purpose is still something expected to happen after that action.

Yes. פצע is a masculine singular noun, so the matching pronoun is הוא, which in English is usually translated here as it.

So הוא does not mean a male person here; it is just grammatical agreement with פצע.

It can often be omitted:

• כדי שיישאר נקי = also possible

Including הוא makes the subject more explicit and can sound a bit clearer.

Because נקי agrees with פצע.

• פצע = masculine singular
• therefore נקי = masculine singular

If the noun were feminine, the adjective would change:

• היד נשארה נקייה = the hand stayed clean

Even though נקי comes after the verb יישאר, it still has to match the subject in gender and number.

Not naturally in this sentence, because the subject of the purpose clause is different.

• Main action: I disinfected
• Purpose/result: the wound stays clean

When the subject changes, Hebrew usually prefers כדי ש + finite verb:

• כדי שהוא יישאר נקי

If the same subject did both actions, Hebrew could often use כדי + infinitive:

• הלכתי לבית המרקחת כדי לקנות פלסטר = I went to the pharmacy to buy a bandage

So in your sentence, כדי ש is the right choice.