Learn Finnish: Minun täytyy pakottaa itseni, koska tavoite on vaikea. - I have to force myself, because the goal is difficult.

Itseni means "myself". It is formed by combining the reflexive pronoun itse (self) with the first-person possessive suffix -ni (my). Because the logical subject of the sentence is "I", the reflexive pronoun must match it with the -ni ending.

Why does the sentence start with Minun instead of Minä?

The verb täytyy (must / have to) is a "necessity verb". In Finnish, these verbs require their subject to be in the genitive (possessive) case, which is why minä (I) becomes minun.

Normally, the object of a täytyy sentence is in its basic form without an -n ending. Does itseni break this rule?

No! The -n you see is just the first letter of the possessive suffix -ni. When you add a possessive suffix to a word, both the basic form (nominative) and the -n form (genitive) of itse end up looking exactly the same: itseni. It fits the rule perfectly without you having to worry about which case it is.

If I want to say "I force" in a normal sentence, how does the verb pakottaa change?