Learn Finnish: Silkkinen huivi on todella kallis lahja. - A silken scarf is a really expensive gift.

The base noun is silkki (silk). By adding the -inen suffix, you create an adjective describing what something is made of or what its properties are. This makes silkkinen mean 'silken' or 'made of silk'.

How would the word silkkinen change if we were talking about multiple scarves?

Adjectives ending in -inen have a special stem: the -nen changes to -se- before you add most endings. To say 'silken scarves' (plural nominative), you use the stem silkkise- and add the plural -t, giving you silkkiset huivit.

The word todella means 'really', but I often see tosi used instead. Are they interchangeable?

Often, yes. Tosi (true) and todella (truly/really) can both be used to emphasize an adjective, like todella kallis or tosi kallis. Grammatically, todella is actually the adessive case form of the word tosi (stem: tode- + -lla).

Why is lahja in the basic nominative case here rather than the partitive case?

This is a classic 'A is B' sentence. Because the subject huivi (scarf) is a singular, countable object, the describing noun lahja (gift) must also be in the nominative case to match it. If the subject was an uncountable mass like kahvi (coffee), the describing word would be in the partitive.