Jangan sampai lupa mengunci jendela malam ini.

to forget

Breakdown of Jangan sampai lupa mengunci jendela malam ini.

• jendela: the window
• malam ini: tonight
• jangan sampai: don’t let

Questions & Answers about Jangan sampai lupa mengunci jendela malam ini.

What nuance does jangan sampai add compared to just jangan lupa?

• Jangan lupa = a simple reminder: don't forget.
• Jangan sampai lupa = stronger/urgent: don't let it happen that you forget; implies there could be a bad consequence if you do.

What does sampai mean in this construction?

Literally it means "until/to," but in jangan sampai + clause/verb it means "don't let it reach the point that …" Examples:

• Jangan sampai terlambat.
• Jangan sampai salah kirim.

Do I need untuk after lupa? Should it be lupa untuk mengunci?

Both are correct:

• Jangan sampai lupa mengunci ... (most common and natural)
• Jangan sampai lupa untuk mengunci ... (a bit more formal or careful)

Why are there two verbs in a row: lupa mengunci?

Indonesian often puts verbs back-to-back without a linker. Verbs like lupa, ingat, ingin, harus, bisa can be followed directly by another verb:

• Saya lupa membawa kunci.
• Dia ingat menutup jendela.