Subjunctive form is an attitude, not a defined structure. That is why we call it the subjunctive mood.
The English language has an ill-defined grammatical structure to denote the subjunctive. At least English has a vaguely defined subjunctive structure, than many languages not bothering at all with having even a vague form of subjunctive convention.
Unbeknownst to many who understand the English "subjunctive", the English grammatical term subjunctive is actually only subset of a general subjunctive voice that includes propositional, hypothetical, improvisational, etc. In fact, if you think hard enough, subjunctive situations form a continuum that sometimes could be categorised with difficulty into one of the bucket/pigeon-holes of subjunctives.
That is to say, the English grammatical term subjunctive is not the same as the linguistic term subjunctive.
As a general term, subjunctive is when there exists intention to describe an imaginary situation.
If you understood simple mathematics of real and imaginary numbers, you could say that subjunctive is the imaginary part of the graph. Subjunctive operates in imaginary time. Subjunctive mood shares the same set of temporal tenses with the real mood.
Therefore, regardless what structure you are using, and if you can properly structure your phrase to describe an imaginary situation, you will be speaking in the subjunctive mood, regardless of the tenses used. It just so happens that most of subjunctive situations in our daily English awareness is neatly described by the deployment of past and perfect tenses.
Notice the continuum of "imaginariness", which is sometimes difficult to quantize into grammatical buckets. Like a number can have both real and imaginary parts. So too a predicate. Of course, many "grammarians" perplex themselves into the subjunctivitis obsession of quantization, asking "is this propositional, hypothetical, hyperthetical, english-subjunctive-mood, etc".
- I can love you if you too love me.
- I can love you if you too could love me.
- I could love you if you too could love me.
- I would love you if you too would love me.
- I could have loved you, had you too loved me.
- Could you please love me, if I too love you?
- Would you please love me, if I too loved you?
- Would you please love me, if I too had loved you?
- The can can explode, when placed in heat.
- The can might explode, when placed in heat.
- The can could explode, if placed in heat.
There is an advantage in having even a vague subjunctive convention, like English does. It allows English speakers to project ourselves in abject courtesy and humility with fewer words. As my oft example illustrates ...
- Could you remove your shoes in the porch before coming in?
The speaker is saying,
I don't want to presume that it will be a reality. I am hopeful, but I am prepared to accept that you might not do it. But, I do prefer you taking your shoes off in the porch, hoping for the reality that you could see the disfavour the snowy slush from your shoes would be doing to my living room.
Courtesy as a subjunctive mood is one aspect that non-native speakers of English find difficult to understand, which most of us take for granted.