Why is the particle omitted in this relative clause?


I am doing [this](https://www.wasabi-jpn.com/japanese-lessons/japanese-
grammar-exercises-relative-clauses/) exercise from wasabi-jpn.com. The
exercise includes this sentence:

[code]

    The sweater (I) bought last week was expensive.
    
[/code]

Apparently, the correct answer is

[code]

    先週買ったセーターは高かったです。
    
[/code]

I realize that the particle は cannot be used after 先週 because it's a relative
clause, but why there is no particle at all?




Firstly, using the particle が would make no sense at all. が marks the
**subject** of a verb, so 先週 **が** セーターを買いました would mean that "last week
bought a jumper". Unless you have some bizarre world where you have
anthropomorphised the weeks then this sentence is pure gibberish.

は and が are not interchangeable ([see
this](https://japanese.stackexchange.com/questions/22/whats-the-difference-
between-wa-%E3%81%AF-and-ga-%E3%81%8C)). は **can** mark a subject, but it does
not do so in this case. The subject in this sentence is the hidden/implied
"I".

Adding は to 先週 denotes that either 'last week' is the topic of the sentence,
or that you are contrasting last week with some other time [see this
post](https://japanese.stackexchange.com/questions/28017/when-do-you-
mark-%E3%81%8D%E3%82%87%E3%81%86-or-other-similar-relative-time-expressions-
with-%E3%81%AF).

Finally, you are right that は is not normally allowed in a relative clause,
but there is an exception. は can be retained if it is used as a contrast
marker rather than as a topic (see [this
post](https://japanese.stackexchange.com/questions/54689/%E3%81%AF-
vs-%E3%81%8C-in-subordinate-clauses/54693#54693)).

In conclusion, no particle is needed in either the full sentence above or in
the sentence which is just the relative clause.




Firstly, using the particle が would make no sense at all. が marks the
**subject** of a verb, so 先週 **が** セーターを買いました would mean that "last week
bought a jumper". Unless you have some bizarre world where you have
anthropomorphised the weeks then this sentence is pure gibberish.

は and が are not interchangeable ([see
this](https://japanese.stackexchange.com/questions/22/whats-the-difference-
between-wa-%E3%81%AF-and-ga-%E3%81%8C)). は **can** mark a subject, but it does
not do so in this case. The subject in this sentence is the hidden/implied
"I".

Adding は to 先週 denotes that either 'last week' is the topic of the sentence,
or that you are contrasting last week with some other time [see this
post](https://japanese.stackexchange.com/questions/28017/when-do-you-
mark-%E3%81%8D%E3%82%87%E3%81%86-or-other-similar-relative-time-expressions-
with-%E3%81%AF).

Finally, you are right that は is not normally allowed in a relative clause,
but there is an exception. は can be retained if it is used as a contrast
marker rather than as a topic (see [this
post](https://japanese.stackexchange.com/questions/54689/%E3%81%AF-
vs-%E3%81%8C-in-subordinate-clauses/54693#54693)).

In conclusion, no particle is needed in either the full sentence above or in
the sentence which is just the relative clause.

