Learn Arabic (Modern Standard): إياك والشفرة الحادة حينما تقطع الخشب. - Beware of the sharp blade when you cut the wood.

This is the standard formula for expressing a warning in Arabic: إيّا + pronoun + و + the danger. While it looks like the word 'and', grammarians explain it as linking two hidden verbs. It essentially means '[Save] yourself AND [avoid] the sharp blade.' In English, we simply translate this whole structure as 'Beware of [danger].'

What grammatical case does الشفرة take after this warning structure?

It takes the accusative case (mansub), so it ends with a fatha: الشفرةَ (ash-shafrata). This is because it acts as the direct object of that implied hidden verb 'avoid'. Consequently, the adjective modifying it must also be accusative: الحادةَ (al-ḥāddata).

Could I use عندما instead of حينما to mean 'when' in this sentence?

Yes, عندما and حينما are completely interchangeable here before the present tense verb تقطع. However, be careful not to use لمّا (lammā) here; although it also means 'when', it is normally followed by a past tense verb in Modern Standard Arabic.

If I am talking to a female, how do the words in this sentence change?

You have to change two words to match the feminine addressee. The warning particle takes a kasra to become إياكِ (iyyāki), and the present tense verb تقطع must take the feminine plural suffix to become تقطعين (taqṭaʿīna).