Learn Dutch: Er zit iets liefs in vaders envelop. - There is something sweet in father's envelope.

This is the rule for partitive adjectives! When an adjective comes after words like iets (something), niets (nothing), wat (some/something), or veel (a lot), it gets an extra -s. The base word is lief (sweet/nice), so it becomes iets liefs (something sweet).

Why does the sentence use zit instead of is?

When something is contained inside something else (like in a bag, a box, or an envelope), Dutch prefers to use zitten (to sit) rather than zijn (to be). So Er zit... translates smoothly to 'There is...', but it literally means 'There sits...'.

Could I say iets zoets instead of iets liefs?

Only if you mean sweet as in sugary! In Dutch, zoet describes the taste of sugar or candy. Lief is used for 'sweet' in the sense of kind, lovely, or endearing. Here, it implies a nice message, a card, or a loving gesture.

Why is there no apostrophe in vaders (father's)?