I'll try to make it simple. In Spanish, we have subordinate clauses that start by "que". Examples: -El perro (que vino ayer) es bonito. -La casa (que compró Juan) es grande. -Los libros (que tiene ese hombre) son viejos. We have nouns (perro, casa, libros), so we can attach these subordinate clauses. We can say, as well: -El (que vino ayer) es bonito. / The one that came yesterday... -La (que compró Juan) es grande. / The one that Juan bought... -Los (que tiene ese hombre) son viejos. / The ones that that man has... Now, we omitted the nouns, but we have the feminine or masculine determinants (I don't know the English words for grammar, sorry), so we can understand, by context, what we are talking about. The problem is now, when we want to be neutral, when we don't know what things we are talking about. For example, in English we will say "What that man has are old books" or "The thing that man has are old books". When we want to say "the thing /what", we use "lo que". Examples: -Lo (que tu quieres) es una manzana. -Lo (que compró Juan) es feo. -No me gusta lo (que dices). -Lo (que tiene ese hombre) son libros viejos. "Todo lo (que tienes que saber)" is the same. Why we say "lo"? Because "todo" is not a noun here, it's an adverb. We could say "Todo aquello (que tienes que saber)" or "Todas las cosas (que tienes que saber)", but we can't say "Todo * (que tienes que saber)", because the determinant is missing.

Si dices "tengo entendido", "tengo pensado", etc. (no creo que haya muchas más frases construidas de ese modo), es porque no lo has entendido ni lo has pensado en el momento de hablar. Me explico mejor con un ejemplo: -¿Qué harás este invierno? -Tengo pensado ir a mi pueblo. Eso significa que no has decidido en ese momento ir a tu pueblo, sino que ya estaba decidido desde hacía tiempo. "He pensado ir a mi pueblo" sería como "I've been thinking...", mientras que "Tengo pensado ir a mi pueblo" sería como "My plan is..." El caso de "Tengo entendido", expresas que has oído algo, pero hace tiempo. Ejemplo: -¿Dónde está Ana? -Tengo entendido que se fue a Canadá. Eso significa que te contaron o has oído que se fue a Canadá, pero te lo contaron o lo oíste hace tiempo. También indica inseguridad, es decir, "Tengo entendido que..." quiere decir que no sabes si realmente se fue, sólo que te lo contaron así. Espero haberte ayudado.

In Spanish we can have sentences without a verb! Let me explain. Maybe you have stumbled upon the common expression "¿Qué tal?". There's no verb. Some people say "Yes, there is, but it's omitted". Maybe. But we understand that sentence and there is no explicit verb. The sentence "Muerto mi padre cuando yo tenía nueve años" normally wouldn't be alone, and I bet it's followed by another sentence, separated by a comma. That's because in latin you could do especial constructions like that, called "Ablative absolute". In english it would be something like (and I know this is grammatically wrong, but maybe you will get the meaning by analogy): " *Done these things, the king and his wife left the castle" " *Dead my father when I was 15, I had to get a job" etc. There's a continuity meaning, but not by its own, it needs another sentence. It's, anyway, somewhat literary. You wouldn't hear somebody speaking like that on the bus.

When you talk about a person, you always say "a" if it's a direct object. For example, you wouldn't say "Odio los niños", that sounds weird, it should be "Odio a los niños". You wouldn't say either "Quiero Juan", you would say "Quiero a Juan"; but, of course, you should say "Odio los lunes" or "Quiero agua", because they are not people. (It's a little bit more difficult than this, but that's the general rule!)