On the fijo/seguro topic, I personally wouldn’t use fijo as creo
Though fijo can also be the first person singular present form of the verb fijar (“to fix in place”), do not mistake the expressions I mentioned as introduced by a verb. Fijo acts as a noun (“fixed”) in them.
Let’s take for example this one:
Fijo que mañana llueve.
I’m sure tomorrow it will rain.
It’s only coincidental that you can swap in creo (present of creer, “to think/believe”) por fijo:
Creo que mañana llueve.
I think tomorrow it will rain.
We could certainly ask:
¿Crees que mañana llueve?
Do you think it will rain tomorrow?
But we can’t conjugate fijo for tú:
*¿Fijas que mañana llueve?
Instead, we leave the noun as is:
¿Fijo que mañana llueve?
Is it a sure thing it will rain tomorrow?
I hope this clears things up.
Originally published in Talk like a Spaniard.