Because the form "leest" (a conjugation of the verb "lezen", to read ) tells us that "zij" must be singular in this sentence. They read the newspaper would translate to Dutch as "Zij lezen de krant".
See here for some more on Dutch conjugation. Unfortunately, "lezen" is slightly irregular:
- Ik lees - I read
- Jij leest - You read
- Hij/Zij/Het leest - He/She/It reads
- Wij lezen - We read
- Jullie lezen - You (plural) read
- Zij lezen - They read
Yes. But you must pay attention to the verb.
'Zij leest' = 'She reads/is reading'
'Zij lezen' - 'They read/are reading'