"Lezen" is indeed the infinitive form, but also the indicative (regular) form for plural subjects. Here's the conjugation table for the present simple:
- Ik lees - I read
- Jij leest - You (singular) read
- Hij/Zij/Het leest - He/She/It reads
- Wij lezen - We read
- Jullie lezen - You (plural) read
- Zij lezen - They read
Though it may not look like it, "lezen" is a regular verb (in the present simple, at least) and follows the conjugation rules listed here. The changes you see above, like the z switching to an s and the doubling of the vowels, are due to the Dutch spelling rules.
"Laeser" is definitely not a Dutch word. The "ae" diphthong is not used at all in modern Dutch.
Jongen (boy) is the child version of man (man). I dont think duo knows other vorms, but you can say: jongeman (young man) kerel (Guy, more for man), vent (also guy like). Put "tje" behind a word and you make it "smaller" for example: kereltje (little guy) or ventje (also little guy)
I'm confused. Everywhere I check s and z in Dutch are alveolar fricatives. But in a lot of words I listen to they definitely sound different (including the TTS here). It sounds a bit like post-alveolar (ʃ and ʒ) or even retroflex (ʂ and ʐ). They are especially almost indistinguishably similar to the Slavic consonants (like Polish sz and ż since they are laminal and softer than the audios on Wikipedia). I'm surprised that I didn't find any comment about how weird they sound. I couldn't find anything explaining it. How should I pronounce them? Why do they sound different?