For example, if I say "Supposons qu'il y ait [qch], cela signifie que ...", where "cela" refers to the "[qch]", then should I use the subjunctive of "signifier"?
2 Answers
You can use the conditional, not the subjunctive.
- Supposons qu'il y ait des erreurs, cela pourrait/
puisseavoir de graves conséquences.
You can also use the indicative in the present.
- Supposons qu'il y ait des erreurs, cela peut avoir de graves conséquences.
No, you don't use the subjunctive for signifie.
"Cela signifie..." is a main clause. You could change the comma in your sentence for a full stop:
Supposons (main clause 1) qu'il y ait [qch] (subordinate clause). Cela signifie (main clause 2) que [...]
Most of the time, subjunctive occurs in a subordinate clause, not a main clause. You could however use conditional (cela signifierait que) to mark hypothesis.