This is the city Obama was born.

[keannu]

Can "in which" or "where" be omitted in this structure?
gz52
ex)This is the city (in which) Obama was born.
=>This is the city Obama was born.

[Raymott]

Can "in which" or "where" be omitted in this structure?
gz52
ex)This is the city (in which) Obama was born.
=>This is the city Obama was born.

No.

[HanibalII]

Can "in which" or "where" be omitted in this structure?
gz52
ex)This is the city (in which) Obama was born.
=>This is the city Obama was born.

Yes, 'in which' can be omitted. However, if you omit 'in which' you must include 'in' at the end of the sentence in order to clarify that it was the city he was born in.

[Raymott]

Yes, 'in which' can be omitted. However, if you omit 'in which' you must include 'in' at the end of the sentence in order to clarify that it was the city he was born in.

In that case, you're not omitting "in which". You're omitting "which" and moving "in" to the end.
It's certainly possible to change "That's the city which Obama was born in" to "That's the city Obama was born in." That's a rather common omission of the relative pronoun. You can't completely omit "in which". You also can't omit "where", because it means "in which".

[HanibalII]

In that case, you're not omitting "in which". You're omitting "which" and moving "in" to the end.
It's certainly possible to change "That's the city which Obama was born in" to "That's the city Obama was born in." That's a rather common omission of the relative pronoun. You can't completely omit "in which". You also can't omit "where", because it means "in which".

Yes, but I think Keannu was referring to the use of 'in which' as paired. Which is why I said as a pair, it can be omitted, but that he would have to include an 'in' at the end.

[keannu]

What is this? My grammar books says if an antecedent is a general word such as "time" "place" "reason", relative adverbs can be omitted. Is this an exceptional case?

ex)This is the place a theater used to be.

[keannu]

Maybe only when you use "place", "where" can be omitted, right?

[Raymott]

Maybe only when you use "place", "where" can be omitted, right?

Yes, that sounds right. You can say, "This is the place Obama was born", but not "This is the city Obama was born".
If in doubt, use the relative pronoun.