Read the mind of Buster the dog and complete his inner monologue.

I wish I could catch that fluffy squirrel taunting me from the oak tree! If only this invisible forcefield of a glass door were open, I would definitely show him who the good boy really is.

I wish I could catch that fluffy squirrel...

We use "wish + could + base verb" to express a desire for an ability or possibility that we don't currently have.

If only this invisible forcefield of a glass door were open...

We use "if only + past simple" for present unreal situations. In formal English and the subjunctive mood, "were" is used instead of "was" for all subjects (though "was" is often heard in casual speech, "were" is the standard grammatical choice here).