The subjunctive is used to express ignorance or doubt.
Here is a list of common expressions that introduce an aspect of ignorance or doubt to the sentence, and therefore trigger the use of the subjunctive.
| dudar que ... | to doubt that ... |
| es dudoso que ... | it is doubtful that ... |
| es improbable que ... | it's unlikely that ... |
| es incierto que ... | it's uncertain that ... |
| es posible que ... | it's possible that ... |
| no creer que ... | not to believe that ... |
| no es cierto que ... | it's not certain that ... |
| no estar convencido de que ... | to not be convinced that ... |
| no estar seguro de que ... | to not be sure that ... |
| no parecer que ... | to not seem that ... |
| no pensar que ... | to not think that ... |
| no suponer que ... | to not suppose that ... |
| puede ser que ... | it may be that ... |
| negar que ... | to deny that ... |
| no es verdad que ... | it's not true that ... |
| no es cierto que ... | it's not certain that ... |
| no imaginarse que ... | to not imagine that ... |
| temer que ... | to suspect that ... |
Remember, there are also expressions that trigger the use of the indicative mood, since they introduce a quality of certainty.
Here's a partial list of such expressions.
| no dudo que ... | to not doubt that ... |
| estar seguro de que ... | to be certain that .. |
| creer que ... | to think that ... |
| no negar que ... | to not deny that ... |
| es verdad que ... | it's true that ... |