Certainty
How sure is the speaker that this is true?
Meaning
Certainty shows how sure the speaker is that a statement is true. The speaker can present something as certain, likely, or doubtful. Languages use words, verb forms, or tone of voice for this, and some have no special certainty form. How the speaker knows concerns the source of the information, not how strongly the speaker believes it.
How to spot it
Look for what makes a statement sound certain, likely, or doubtful, rather than simply describing what happens.
In your native language
English uses words such as definitely, probably, and maybe to show different levels of confidence. Must can express a strong conclusion, while may, might, and could express uncertainty about what is true. In this use, must does not give an order and may does not give permission.
The lights are on. Anna must be home.
Must expresses a strong conclusion, not a duty.
Ben might be on the bus.
The speaker presents this as possible, not certain.
They'll probably arrive before lunch.
Probably presents the arrival time as likely, but not certain.
In your learning language
Spanish expresses certainty with words and phrases such as seguro que, probablemente, and estar seguro de que. Positive certainty expressions normally take the indicative, the forms used to state facts, while expressions such as no estoy seguro de que often take the subjunctive (subjuntivo), presenting the statement as uncertain. The future tense can also express a guess about the present rather than a future event.
Seguro que Marta está en casa.
I'm sure Marta is at home.
No estoy seguro de que la tienda esté abierta.
I'm not sure the store is open.
Esté is a subjunctive form following an expression of uncertainty.
Luis estará en el trabajo a estas horas.
Luis is probably at work at this time of day.
The future form estará expresses a guess about the present.
Learners whose language does not change verb forms after expressions of doubt may overlook the subjunctive after no estoy seguro de que. Learners whose future forms refer only to later events may misunderstand estará when it is a guess about someone's current location.