Both the 是…的 (shì…de) construction and 了 (le) can both be used to talk about
past events in Mandarin Chinese grammar. Here’s how to use the two constructions
correctly, as they are different.
是…的 (shì…de): focus on the circumstances
A quick recap of the 是…的 (shì…de) construction.
是…的 is used to talk about events that happened in the past:
是…的 can also emphasise some information in a sentence (that describes a past
event):
The 是…的 construction describes past events, and can also emphasise particular
details of a sentence such as time, manner or place.
Because 是…的 emphasises the circumstances of past events, it’s useful for
describing how, where or when something happened.
An example of using 是…的 to describe how something happened:
An example of using 是…的 to describe where something happened:
An example of using 是…的 to describe when something happened:
The most important point is that 是…的 is suitable for describing the
circumstances or manner of things in the past.
了 (le): focus on completion
Unlike 是…的, 了 (le) is not necessarily about the past. See this 了 (le) grammar
summary for more details.
了 has many uses, one of which is about marking completeness of actions. 了 is
commonly misunderstood as being about the past tense, but 了 is not about the
past tense (Chinese grammar does not really have tenses).
Some examples using 了 that show how it is about completeness of an action, or a
new state:
Other than not necessarily being about the past, 了 is also different to 是…的 in
that it cannot be used to describe the circumstances of an action, only the
completeness.
Comparing 是…的 and 了
Compare these sentences that are the same other than the use of 是…的 or 了:
Both describe an event in the past. The 了 sentence only indicates that the
action was completed in that timeframe, whereas the 是…的 sentence emphasises
the circumstances (the time it happened).
You can also see the difference if we make those sentences into questions:
The difference seems subtle, but pay attention to the re-phrasings that highlight
where the emphasis is.
With “你昨天买了面包吗？”, it seems that buying bread might be a regular occurrence, or
at least that the asker expected you to have bought bread yesterday. They are
primarily asking whether or not this action was completed at all, and the time
is just a bit of context for the question.
With “面包是昨买的吗？”, the fact the bread was purchased is already an established
fact. The asker wants to know specifically whether it happened yesterday and not
some other day. Perhaps the asker wants to know how fresh the bread is, so they
use 是…的 to focus on the circumstances of the action, in this case the time.