As they have already pointed out, even some native speakers get it wrong quite often. So don't worry if you can't get it. It is just a matter of learning it verb by verb and getting used to it. The easiest way for us to find out the correct form is to replace "que" and everything after it by the word "eso" (that) or "esto" (this). In your example, it would be: "Estoy feliz de eso". Visualizing the sentence in that way makes it obvious, at least for natives, that the word "de" is totally necessary there, because "Estoy feliz eso" doesn't make any sense. Probably it is not gonna be that obvious for you at first, but give it a try and try to figure it out this way. I'm sure it is gonna be useful.

Hello! If you use the second phrase you would be committing "queísmo". "Queísmo" is the undue suppression of a preposition (usually "de") before the conjunction "que", when the preposition is required by some word of the statement. When a sentence has an adjective accompanied by a prepositional complement you should use "de que". For example: Estoy seguro de que llegarás lejos (I'm sure you'll go places) A prepositional complement complements or specifies previous information that is too general and imprecise.

This is a toughie but I will try simplify matters. This is called 'dequeismo' according to 'La Real Academia de la Legngua Española'. The rules and exceptions are complex even for us. We native speakers of spanish always get it wrong. As a general rules avoid using 'de que' and just stick to 'que' and you will be safe on most occasions. There are more instances when just 'que' sounds better and is grammatically correct. Estoy contento que llegues de viaje (fine) Estoy contento de que llegues de viaje (Hmm, of dubious syntactic sense). (To my ears it denotes a bit of a rough background, lack of education, etc) I hope it helps. Cheers mate!

Es muy sencillo, Matt: Hay algunos verbos que necesitan la preposición "de" y hay otros verbos que no la necesitan. El problema es saber cuales son esos verbos:( Aquí puedes ver unas listas de los dos tipos de verbos: http://hispanoteca.eu/Foro-preguntas/ARCHIVO-Foro/Verbos%20con%20preposici%C3%B3n.htm Y también te puede ayudar este diccionario de uso de las preposiciones: https://vk.com/doc4827744_364022027?hash=bbdc62c13e414c3a00%3Edl=2ce7d5c5f6fa8bdb84 https://vk.com/doc4827744_364022667?hash=9b7b9643d3f0e848c2%3Edl=0ef077e7d555de3fb7 Yo suelo estar dudando también con los "English collocations": go in, go down, go back...too many directions!