In all of these sentences, the key to knowing whether to use a clause with a verb in the
condicional or the pasado de subjuntivo--or more likely, the
imperfecto de subjuntivo, just as you wrote in your title: "imperfect subjunctive"--is the position of
si 'if'. The situation is quite analogous to English, where the pattern is
X if Y or If Y,
X; the verb in the "Y" clause, following if, is technically in the present or past subjunctive, which is almost always identical to the normal simple present or past tenses, respectively, in Modern English except for a few moribund relics like if I were rich, informally if I
was rich, just like the normal past indicative I was rich.
The verb in the "X" clause normally contains would (or its contraction
'd) plus the base form of the following verb, a construction that is indeed often referred to as "the conditional tense" (cf. Spanish
Putting this all together, we can say My life would be awful
if I had cancer or If I
had cancer, my life would be awful. Very similarly, in Spanish, illustrated in your partial sentence 1., since
si occurs after the first clause, whose verb is therefore the condicional of
ser, namely sería, the verb in the si-clause must be in the
imperfecto de subjuntivo, like tuviera, the i. s. of
tener, as in yo tuviera cáncer 'I had cancer'.
Compare this with your partial sentence 3. which begins with Si and therefore the verb in its clause is the
imperfecto de subjuntivo form estuviera. This means that you'll need a following main clause whose main verb must be in the
condicional, like in Si estuviera de vacaciones, agradecería a Dios 'If I were/was on vacation, I
would/I'd thank God'.