Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

(1) Whether the main verb is (a) primary or (b) secondary.

(2) Whether the dependent verb is to denote completed action (i.e. past with reference to the main verb) or incomplete action (i.e. present or future with reference to the main verb). Then—

[*] a. If the leading verb is primary, the dependent verb must be in the Present if it denotes incomplete action, in the Perfect if it denotes completed action.

[*] b. If the leading verb is secondary, the dependent verb must be in the Imperfect if it denotes incomplete action, in the Pluperfect if it denotes completed action:—

1. (1) He writes [primary] to warn [incomplete action] us, scrībit ut nōs moneat.
2. I ask [primary] what you were doing [now past], rogō quid fēceris.
3. (2) He wrote [secondary] to warn [incomplete] us, scrīpsit ut nōs monēret.
4. I asked [secondary] what you were doing [incomplete], rogāvī quid facerēs.

[*] c. Notice that the Future Perfect denotes action completed (at the time referred to), and hence is represented in the Subjunctive by the Perfect or Pluperfect:—

1. He shows that if they come (shall have come), many will perish, dēmōnstrat , sī vēnerint , multōs interitūrōs.
2. He showed that if they should come (should have come), many would perish, dēmōnstrāvit , sī vēnissent, multōs interitūrōs.