subjunctive mood

Hello, now i am learning the Subjunctive mood. I made a test online. And i made some mistakes but i did not understand whrere is the problem. Please help me.
"We have been walking for two hours. I am so tired. If only we ..... a taxi!"
had found
should find
could find

i chose "had found" because i decided that there is the subjunctive mood II , but the system corrected it and wrote "could find"

Why? please help to understand

Perosnally I would select "could find" as this would seem to imply that you are looking for the option to take the taxi. You could find a taxi or you could keep walking or could do anything else if there was an oppotunity.

not being a language expert either, my explaination may not be correct but the option would be the right one to go with.

Chris

subjunctive mood

Hello, now i am learning the Subjunctive mood. I made took a test online. And I made some mistakes but i did not understand where is the problems are. Please help me.

"We have been walking for two hours. I am so tired. If only we ..... a taxi!"
had found
should find
could find 

I  chose "had found" because i decided that there is the subjunctive mood II, but the system corrected it and wrote substituted "could find" 

Why? please help to understand

It's because of the tense of 'have been walking'  (present tense is required). If instead it had been 'had been walking' then 'had found' would be correct, being past subjunctive.