Limiting with 〜だけ
Grammar: Degree » だけ
Usage with particles
When the word marked by だけ is the subject, the が may be omitted. Likewise for the object and を. When not ommitted, だけ only appears before が or を, not after.
If the word marked by だけ has the particle で, だけ may appear before or after で. This also applies to に and と.
English
In English, だけ is often translated “only” or “just”. These make sense in an English translation, but a learner of Japanese should be aware that these translations obscure its meaning within a Japanese sentence.
Related Grammar
Examples
Because it's only this road with 〜だけ

Takagi convinces Nishikata to spend his summer practicing riding his bike with her on the back.
One day, she asks Nishikata if he’s progressed on his summer homework, saying they should go to his house to work on it.
Since the road back is wide and without cars, Takagi convinces Nishikata to ride doubles to his house.
- 西片:
- 「こ…この道だけだからね。」
- “Because it's only this road.”
Nishikata says he’s riding doubles because it’s limited to 「この道」, this road.
Don't suspect only me with 〜だけ

In a snow-covered mountain villa, a room full of suspects listens as Detective Mouri explains how their host was murdered. The accused notes she has a receipt showing she was at the convenience store at the bottom of the mountain when the murder took place.
- 中原:
- 「それにまだ金澤先生のアリバイもあやふやよ!!」
- “Moreover, Mr. Kanazawa's alibi is also vague!”
- 「私だけを犯人扱いしないでよ!!」
- “Don't treat only me as the suspect!”
だけ limits the object being treated as a suspect to 「私」.
See Also
Only for a little bit with 〜だけ

While waiting for his brother, Akatsuki finds Akari. He decides to pass the time by having Akari give him a tour of the area, but she explains that as a trainee she can’t take on customers by herself.
Akatsuki hops into Akari’s gondola, saying it’s okay as long as he’s not a customer; he’s a friend.
- 灯里:
- 「本当にちょっとだけですよー」
- “It's really just a little bit!”
Akari is limiting the the gondola tour to 「ちょっと」, a little bit.