Learn Finnish: Hän uskoo, että ystäväni valehtelee. - He believes that my friend is lying.

In Finnish, you must always put a comma before the conjunction että (that) when it connects a main sentence to a subordinate clause. This is a strict punctuation rule, unlike in English where the word "that" usually doesn't need a comma.

Where is the word for "my" in ystäväni?

The -ni at the end of ystäväni is the possessive suffix meaning "my". In standard Finnish, you can often leave out the pronoun minun (my) entirely and just attach the possessive suffix directly to the noun.

The translation says "is lying", but doesn't Finnish have an "-ing" form?

Finnish doesn't have a separate present continuous tense like English does. The single present tense verb valehtelee covers both "lies" (in general) and "is lying" (right now). You just rely on the context to know which one makes sense!

The dictionary form of "to lie" is valehdella. How does it become valehtelee?

Valehdella is a Type 3 verb. To conjugate it, you drop the -la ending and add an -e- to get the stem. It also undergoes a consonant change (called consonant gradation) where the weak hd flips to the strong ht, making the stem valehtele-. Finally, for the "hän" (he/she) form, you double the last vowel, giving you valehtelee.