sedang ... ketika ...

In Indonesian, verbs do not change their form to show time. Instead, we use aspect markers to show how an action relates to time.

1. The Background Action (sedang)
The word sedang is placed before the verb to show that the action is in progress at the time being discussed. It works with both root verbs and prefixed verbs.

2. The Stative Restriction
While sedang can be used with adjectives to emphasize a temporary state or feeling (like sedang sakit or sedang marah), its compatibility depends on meaning. Sedang tahu is not the ordinary way to say that someone knows something; use tahu. Do not extend this to all feelings or desires: sedang ingin can describe a current desire, and standard grammar accepts this combination.

3. The Interruption (ketika)
The conjunction ketika introduces the reference time or event and is standard Indonesian. In an interruption example, this is often a punctual action, sometimes paired with tiba-tiba (suddenly). However, ketika does not itself mean that the background action stops, and its clause may also contain an aspect marker when the meaning calls for one. While saat is a common synonym, you will also often hear waktu or pas used in informal speech.

4. Clause Order and Punctuation
You can place the ketika clause at the beginning of the sentence. When you do this, you must use a comma to separate the two clauses (e.g., Ketika dia datang, saya sedang makan). However, if the main clause comes first, no comma is needed (e.g., Saya sedang makan ketika dia datang).

5. Register Differences

• Formal: Use sedang and ketika.
• Casual: In daily conversation, Indonesians use lagi instead of sedang, often paired with pas or waktu instead of ketika (e.g., Aku lagi makan pas dia datang, noting the shift to casual pronouns like aku). Additionally, in casual speech, waktu or pas can be followed directly by a noun to indicate the interrupting event (e.g., Aku lagi tidur waktu gempa). Ensure lagi is placed before the verb, because placing it after the verb means 'again' (e.g., makan lagi means 'eating again').
• Literary: In written or highly formal contexts, you might see tengah used instead of sedang (e.g., Ia tengah membaca buku ketika...).

6. Tense Flexibility
Because Indonesian has no grammatical tense inflection, this pattern can describe the past, present, or future. The timeframe is established entirely by context or by adding time markers. For example, to establish a past timeframe, you can add markers like kemarin or tadi (e.g., Saya sedang makan ketika dia datang tadi). For future events, a future time marker can locate the action (e.g., Besok ketika kamu tiba, saya sedang tidur). The combination akan sedang is also possible when explicitly presenting an action as ongoing at a future reference time; it is attested in Tata Bahasa Baku Bahasa Indonesia. It is not obligatory whenever English uses “will be doing.”