In Spanish "lo" is a neuter article that can be used with adverbs and adjectives (not nouns) followed by "que", to express surprise/admiration/disappointment about how something/someone performs an action or about what someone/something is like.
Have a look at the following examples with:
Notice how, being adverbs, they don't agree with the subject. Be careful with adverbs that take the form of their adjective (e.g., rápidamente=rápido, lentamente=lento). In these cases we will use the masculine singular form of the adjective.
This would be incorrect:
"Lo lenta que anda tu abuela"
Now have a look at some examples with:
Notice how in the examples above, used with adjectives, these agree with the subjects in gender and number (simpático/chico, seria/chica, verdes/campos, altas/hijas).
It is sometimes difficult to know when a word is an adverb or an adjective, but as a general rule, in this type of sentence you will know that it is an adjective if the verb in the sentence is "ser", "estar" or "parecer"." So in these cases the adjective will agree in gender and number with the subject.
This type of structure isn't always in the form of an exclamatory sentence, as in the above examples; it can also be a subordinate clause. Have a look:
It is also used with ordinal numbers, not necessarily to express surprise/admiration/disappointment, as with the examples above, but when simply meaning: "the first thing that ....."the second thing that ...":
See also Using neuter article Lo + adjective + ser in Spanish and Lo que + verb + subject for emphasis
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