En ciertas oraciones no sé si es más correcto usar que o de que. ¿Cuáles son las reglas para utilizar que/de que?
Ejemplos:
Estoy seguro que me fue bien.
Estoy seguro de que me fue bien.
In some sentences I don't know when it is more correct to use que or de que. What are the rules for using que/de que?
Examples:
Estoy seguro que me fue bien.
Estoy seguro de que me fue bien.
There is a simple rule I learned in high school (oh so many years ago) and stuck with me.
You need to replace que and everything after it with eso (which means it), then you will easily see which one is wrong.
Estoy seguro eso (I'm sure it), is wrong.
Estoy seguro de eso (I'm sure of it), is correct.
This helped me from the moment I learned it, up until now that I've written this answer :)
The proof of Juan Manual works perfectly but I want to explain why its works.
Some words (can be nouns, adverbs, etc) needs a preposition ("de") before the relative ("que"). The relative, as you maybe know, it's a referencial to the first element that you want modify with a proposition. I'll be more clear with an example:
Estoy seguro de aprobar.
If you replace "aprobar" (that works as a noun) with a proposition:
Estoy seguro de que aprobaré.
You must use "que" because it's the relative of the proposition. So, both of the following phrases are wrong:
Estoy seguro aprobar.
Estoy seguro que aprobaré.
But, too many people in spanish don't "mistake" this. When someone really do a "queismo", if almost because is an overcorrection, they fix a good way to say it.
Returning to the topic, the real proof for an advanced speaker is replace the proposition by a functional word as some pronoun, but the real proof is split the main sentence and detect the proposition and the relative.
In this context you want to make a mental note of whether que is being used as a relative pronoun or a conjunction.
First, let's define what these terms mean:
Relative Pronoun: Introduces a clause that provides more information about a noun. In Spanish, we often just use que in such sentences
Conjunction: Joins two complete sentences (subject + verb) or a clause. In Spanish, we can use de que in these sentences. More common-place Spanish conjunctions include pero, y, o, sin embargo, etc.
So, for an English speaker this technique will help you choose:
If the English translation of the sentence still makes sense by substituting the English relative pronoun "which" for "that". Use que. If substituting which makes no sense, the correct Spanish usage is de que
So, in these examples:
(incorrect) Estoy seguro que me fue bien => I am sure which it went well for me
(incorrect) Estoy seguro que aprobaré => I am sure which I will approve
Are incorrect because using which doesn't make sense in the English translation. So, I know I need to use de que. These phrases require a conjunction, not a relative pronoun!
(correct) Estoy seguro de que me fue bien => I am sure that it went well for me
(correct) Estoy seguro de que aprobaré => I am sure that I will approve
source: https://www.thoughtco.com/que-following-a-noun-3079287
In Mexican Spanish, que means either "what" or "that" as in a preposition; de que means "of what", as in origin of something and is more of a directional designation of the origin of an object.
