The past subjunctive form of 'be' is 'were'. The past tense indicative form of 'be' is 'was. From this we can safely say that 'was' is no subjunctive.
Pure modal auxiliaries have one (others say two) forms. When no change takes place in form, we can only guess at from the meaning whether subjunctive is used.
As I have already suggested, there are several factors that play a role in the speaker's choice of inflectional system.
Retired English Teacher