I mean the reason why that part is different from English. I understand that it may be used as a marker for changing subject but I don't understand why it's required to the extent that removing it will be grammatically incorrect.
There are different cases:
Not in all cases you have to put que.
Pienso + infinitive is used to express an activity you want to do. For example, Pienso ir a la playa este fin de semana
In another cases you have to put it, and in these cases you are expressing an opinion:
Pienso que + infinitive For example, Pienso que ir a la playa este fin de semana estaría bien.
Pienso que + sentece For example, Pienso que la playa es fantástica.
Think that pienso que is like phrasal verb in English. It's always like this, the same way to creo que, opino que, ... and other verbs that express opinion. If not, it will sound really weird to the interlocutor, like something is missing. You need this connector.
It's funny because in Spain, when imitating American Indian accent, we omit connectors like this.
It will not be grammatically incorrect if you remove the "que", but it will have a slightly different meaning. "Pienso que" translates to "I think that", you could similarly say, for example, "Pienso comerme ese pollo.", which translates to "I think (of) eating that chicken.", in this case, the "of" is omitted in Spanish.
The reason is actually rather simple.
In Spanish, you have words that are nouns, adjectives, or adverbs (say, gato or negro or ahora). You also have things that act as nouns, adjectives, or adverbs but are actually entire phrases.
The simplest of these are just known in Spanish as locuciones or phrases, and are more idiomatic in structure.
But the recursive nature of language allows us to use entire clauses (replete with their own independent verbs, subjects, objects, and complements) to take the place of a standard noun, adjective, or adverb.
Take a simple phrase like
(Mañana) viene el (gato) (negro).
The three words in parentheses can be described with a whole phrase. Hence we can say
- gato: tiene cuatro patas y es pequeño y antipático
- negro: parece ébano
- mañana: cuando salga el sol
In the case of mañana, notice that it's hard to express without including a relative pronoun. That's common for adverbial ones. Let's start by replacing mañana with that phrase:
- Cuando salga el sol, viene el gato negro.
Now let's replace the negro. In switching to new clauses, Spanish requires some sort of relative pronoun. With adverbs, it's generally obvious (temporal phrase: cuando, locative: donde, etc). But for adjectives, there might not seem to be a good choice. The “default”, absent any other relative pronoun, is que. So we use that to signal the start of the new clause:
- Cuando salga el sol, viene el gato que parece ébano.
The Spanish speaker knows this functions as an adjective because the que comes where an adjective would be expected and the subject matches (if the noun is being acted upon, more complex rules with prepositions come into play).
With nouns, the same logic applies. If you would expect a noun, and you see/hear que you know to wait for an entire clause before processing it all as a single noun. While the sentence is now rather unwieldy, we get1
- Cuando salga el sol, viene el que tiene cuatro pata y es pequeño y antipático que parece ébano.
Now, let's say I want to discuss the fact that the cat is black. Here, I need to use the phrase parece ébano not as an adjective but as a noun. Here, we simply use the que all by itself without any demonstrative.
- Que parece ébano es obvio.
The whole phrase here is the subject. How did we determine the forms for the verb and adjective? Well, phrases used like this are consider to be neuter. Neuter things are always singular even if combined with y and will always be third person. So singular and third person leads us to es, and singular and neuter leads to obvio (neuter forms of adjectives are identical in Spanish to masculine forms, except for demonstratives).
Now... How does this apply to pienso que?
What comes after pienso normally? A noun. (Pienso algo being as basic as we can get).
But often we think things that are best described with their own entire clause. What is it that you think? You think (something doing something). That bit in English parentheses is a clause. English has a variety of ways to construct it depending on context and verb, but Spanish nicely has one single way — using a full clause that functions as a noun.
So when you say pienso algo, that algo can be replaced with our phrase, for example, the cat being black.
- Pienso algo.
- algo = el gato es negro
But remember, we said all clauses functioning as a noun/adjective/adverb need to have a relative pronoun. Lacking anything better, we use the default, que to head the clause:
- Pienso que el gato es negro.
How can we prove that we're using this as a noun? Let's formulate it as a question.
- ¿Piensas que el gato es negro?
— Sí, pienso eso. — Sí, lo pienso.
Notice the demonstrative and the direct object (singular and neuter). Those can only replace nouns — or noun clauses, which is what we've used.
1. Okay, technically, this el que... is actually described as just another adjective clause. Meh. I think the point is made, nonetheless.
(Battery dying on laptop, posting this before I lose it)
Pienso que is an introduction to a subordinate clause, meaning that an actual full phrase s coming.
Pienso quelos perros se los comen los gatos
That clause in bold can stand on it's own as a sentence, but in this case it is subordinated from a verb. I think you would call this a subordinate verbal clause... ?
When you use prepositions you're not subordinating anything. The use of
que is generally a dead giveaway of a subordinate clause. No matter what the verb.
Me gusta que me das dinero.
Bebo alguna bebida que se llama "Negra muerte"
El jefe siempre hace que todos hagan su propio trabajo.
Creo que los cuellos de las jirafas son demasiados largos.