立即 / 马上 / 当即【lìjí / mǎshàng / dāngjí】

立即, 马上 and 当即 all mean "at once" and go before the verb. 马上 is the spoken word and is loose about timing; 立即 is written and means no delay at all; 当即 is written narrative about the past.

马上 – soon, by the speaker's measure

马上 can stretch to minutes or even days, as long as the speaker feels it is close. With 就要…了 it says something is about to happen, which the other two can't do.

• × 立即就要过年了 → 马上就要过年了

立即 – no delay

立即 says the action follows straight after something else, with nothing in between. It fits instructions, notices and reports. 立刻 is close to it and slightly less formal.

当即 – there and then

当即 tells what someone did at the moment a past event happened. It reports a past action and can't give an instruction or describe a plan.

• × 请当即回复 → 请立即回复
• × 我明天当即去 → 我明天马上去