We have a bunch of options to express doubt.
-quizá, quizás: these two adverbs mean “who knows”. You can use one or the other. Notice that the final S in “quizás” is not a plural ending, is simply the last letter of the word. So we can say “quizás viene mañana” or “quizá viene mañana” (maybe he/she comes tomorrow). It is the same.
Now, pay attention to these two sentences:
“Quizás viene mañana” (maybe he/she comes tomorrow)
“Quizás venga mañana” (maybe he/she comes tomorrow)
As you see, we have the same form in English but two entirely different forms in Spanish. The first one belongs to the Indicative Mood, whereas the second one belongs to the Subjunctive Mood. This is not the topic for today. However, the two sentences have a slightly different meaning. In the first case, I am pretty sure about the fact. In the second one, I am not.
-tal vez: it has the same meaning as “quizá, quizás”, and again, you can use it either with Indicative or Subjunctive.
“Tal vez puedo ayudarte” (maybe I can help you)
“Tal vez pueda ayudarte” (maybe I can help you)
In some other cases, you must use only one verbal form, either the Indicative or the Subjunctive.
-posiblemente: this means “possibly”. Use it with the Subjunctive, because you are not sure at all about the fact.
“Posiblemente tenga tiempo mañana” (possibly I have time tomorrow)
-puede ser: this means “it can be”. Again, use it with the Subjunctive, for the same reason.
“Puede ser que viajemos a Madrid la próxima semana” (it can be that we travel to Madrid next week)
-a lo mejor: this means “in the best scenario” and it is used always with the Indicative, because although we are not totally sure about the outcome, we really want this to happen, as it is the best option for us.
“A lo mejor podemos ir también a Barcelona” (maybe we can also go to Barcelona)
In some books, for example textbooks for GCSE, you will read that in Spanish we express doubt using the Subjunctive. Totally wrong, as you can see. Remember that GCSE Spanish textbooks are mainly wrong in terms of grammar, vocabulary and structures.
