Handri, this rule is only valid in the earliest stages of learning Spanish. Once you start learning more than the simple present, the rule is mostly wrong. The second verb is usually a participle.
Look at your own post: "is conjugated", conjugated is the second verb and is a participle. It is similar in Spanish.
My understanding is , it's "libro", but it's a softer sound than in English. Think of it this way: hold a lit candle very close to your mouth. Say "pig". You will blow out the candle. Now say "perro" (dog). You won't blow out the candle. The 'B' sound is similar. It's technically a bilabial sound, lip to lip. The 'V' in English is a labiodental sound, lip to teeth, but in Spanish "V" is also a bilabial as the "B", but they're both softer than in English. Someone correct me if I'm wrong.
There are hundreds of words and expressions in both languages that have similar meanings, or even identical meanings in some contexts. There is need to and have to and must, say/speak/talk, go/come, want/like, and many more. There is a reason we have these alternatives. We need them to express different meanings or nuances at times. It is best to learn the closest equivalences in Spanish and resist trying to get away with an almost accurate translation.
Hi. I can't fix it nor can anyone else here because we are all just learners like you.
Necesito is spelled wrong. Be careful of your spelling if you are worried about failing.
In Spanish, including the pronoun is optional. Duolingo usually includes it, Spanish speakers usually don't. Necesito means I need; Yo necesito also means I need.
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un is a, uno would be one. i think they are sort of the same, but there is a bit of a difference. a speaker saying "i need to read a book" seems more casual and normal to say, rather than "i need to read one book", as the latter comes off as more urgent and as though you need to read one and only one. but otherwise, yes the same thing.