Learn Finnish: Sormeni on lasissa. - My finger is in the glass.

In standard written Finnish, you don't always need a separate word for 'my'. Instead, you show ownership by adding a possessive suffix to the end of the noun. Here, the suffix -ni at the end of sormeni means 'my'.

If the dictionary word for finger is sormi, why is it sormeni and not sormini?

Many older Finnish words ending in -i change that -i to an -e- when you attach an ending to them. The word sormi follows this rule, so its stem becomes sorme- before adding the -ni suffix.

Can I add the word minun (my) and say minun sormeni?

Yes, you can! In standard Finnish, you can say minun sormeni (my finger). The pronoun minun is optional for the 1st and 2nd person (my and your), but if you use it, you must still include the -ni suffix on the noun.

What is the -ssa at the end of lasissa doing?

This is a quick review of how to say 'in' something. Finnish uses suffixes instead of separate prepositions for locations. The -ssa suffix means 'in' or 'inside', so adding it to lasi (glass) creates lasissa (in the glass).