Final answer:
The phrase 'No es seguro que...' signals the use of the subjunctive mood in Spanish, which is used to express uncertainty or situations that are not factual.
Step-by-step explanation:
The phrase 'No es seguro que...' in Spanish prompts the use of the subjunctive mood because it expresses uncertainty. When we say 'No es seguro que,' we're implying that what follows is not a certain fact, but rather something that might or might not happen. This uncertainty is exactly what the subjunctive mood is used to convey, as opposed to the indicative mood, which is used to state facts, opinions, or to ask questions. Thus, after phrases like 'No es seguro que,' a verb would typically be in the subjunctive form.