I read from a book: Ya he pensado todo lo que tenia que pensar hoy. Why do we need to add ‘lo’ in this sentence? Why not just: Ya he pensado todo que tenia que pensar hoy.?
Singular "todo" can be an adjective, meaning "every" (or "any"), or a pronoun, meaning "all" or "everything":
- Todo pensamiento cuenta (Every thought counts).
- Todo cuenta (Everything counts).
"Lo que" is a nominal relative pronoun equivalent to English "what", while "que" is a relative pronoun equivalent to English "that" ("que" can also be a conjunction, but that is not the point here).
Curiously, while other similar pronouns like "algo" ("something"), "poco" ("little") and "mucho" ("a lot") can be modified by a relative clause introduced by "que", "todo" cannot:
Ya he pensado algo que puedo hacer. (I've already thought of something that I can do.)
Hay poco que puedes hacer. (There is little that you can do.)
Hay mucho que puedes hacer. (There is a lot that you can do.)
With "todo", "que" alone is ungrammatical and "lo que" is required. In this case, "lo que" acts as a noun ("the things that") and "todo" functions as an adjective). This is different from English, where "all" is a pronoun that can be modified by relative "that":
- Esto es todo LO QUE puedo hacer. (This is all that I can do.)
"Lo que" serves as a neuter relative pronoun, usually meaning "that," "what", "it", "that which", "whatever", "the thing", "the things" or "the thing(s) that". It is used here to refer to a general idea and cannot be omitted in Spanish, i.e.
- Ya he pensado todo lo que[=the things that] tenía que pensar hoy(I've thought everything [ ] I had...)
- Ahorraba lo que le sobraba de(she saved whatever[=cualquier cosa, lo que sea, lo que estaba a su alcance] was left over of..)
- Perdió todo lo que(=the things that he/she had in there, in other words, EVERYTHING) había tenido.
- Todo lo que tenemos es aquí y ahora(=that's all there is, nothing more)
- Lo primero en lo que hay que(=The first thing we have to)