When should I use Liest, Lese, and Lesen?
When are Liest, Lese, and Lesen (and whatever other forms "read" I'm missing out on) meant to be used and why are they used that way? In addition, from what I understand, Sie with a capital "S" means you (formal singular) and with a lowercase "s", sie means "they" or "she". So with respect to completing the phrase "Sie ____ Bücher", why does Dulingo use an uppercase S and why is the answer "Sie lesen Bücher" which translates to "They read books" instead of "Sie lese Bücher" which translates to "She reads books" (which is still grammatically correct from my understanding)?
here the conjugation of the verb lesen
ich lese- i read
du liest- you read (informal)
Sie lesen- you read (formal)
er/sie/es liest- he/she/it reads
wir lesen- we read
ihr lest- you (plural) read
sie lesen- they read
2.person formal always uses the same verbform as 3.person plural the only difference is the capital S for 2. person formal (Sie). exception: in the beginnig of a sentence since a sentence always beginns with a capital letter.
sie lese bücher does not exist : lese is 1. person sing.
The verb you use when talking about yourself in German always ends in "-e" (e.g. ich lese = I am reading, ich gehe = I am going, etc.). The same principle goes for "du" (informal you, singular): du liest, du gehst. etc. and for "Sie/sie" : Sie/sie lesen, Sie/sie gehen etc. Hope this makes sense.
Liest is "you read" (singular)
Lese is "I read"
Lesen is either "they read" or "you read" (formal)
They is "sie" with a lowercase, and you (formal) is "Sie" with an uppercase.
This sentence is tricky because the "sie" is at the beginning of the sentence, which means it has to be uppercase. You just have to know the context of the question to get it right. But, your sentence of "Sie lese Bücher" wouldn't be right anyway because it would be "Sie liest Bücher," because liest is he/she/it.
To your example "Sie ____ Bücher": "Sie liest Bücher."=She reads books. "Sie lesen Bücher."=They read books./You read books. (formal, sg and pl possible)
Many German verbs change the stressed vowel in 2nd and 3rd person singular present. They are a part of the so called "starke Verben" (strong verbs?). In general starke Verben change the stressed vowel in past tense.
examples are: tragen -> er trägt (carry, wear), laufen -> er läuft (walk, run), gebären -> er gebiert (give to birth) [very seldom, I suppose only few German know that word.], lesen -> er liest (read), helfen -> er hilft (help), stoßen -> er stößt (push, come across, many meanings), erlöschen -> er erlischt (go out, become extinct, contract: expire).
In general: a->ä, au->äu, ä->i(e), long e->ie, short e->i, o->ö, ö->i. No guarantee for wholeness.
Eine Liste starker Verben habe ich hier gefunden: https://www.deutschplus.net/pages/Tabelle_starker_Verben