Objects After täytyy

In standard active indicative sentences, a singular total object takes the accusative -n ending (e.g., Ostan kirjan - I buy a book). However, in affirmative necessive constructions (where the verb is used impersonally to express necessity), the rules change. First, if there is a specific subject (the person who must do the action), it is in the genitive case (e.g., minun, sinun, hänen). However, the subject can be omitted entirely to express a general obligation (e.g., Täytyy ostaa kirja - One must buy a book). Second, the object rules are as follows:

• Singular Total Objects: Use the endingless form (nominative-like).
  Example: Minun täytyy ostaa kirja - I must buy a book (not kirjan).
• Plural Nouns: Plural total objects already use the nominative plural (-t) in standard sentences, and they remain the same here.
  Example: Meidän täytyy ostaa kirjat - We must buy the books.
• Personal Pronouns & 'Kenet': These are the exception. They always keep their accusative -t ending (minut, sinut, hänet, meidät, teidät, heidät, kenet).
  Example: Sinun pitää kutsua hänet - You must invite him/her.
• Demonstrative Pronouns: Unlike personal pronouns, demonstrative pronouns (se, tämä, tuo) act like nouns and take the endingless nominative.
  Example: Minun täytyy ostaa se - I must buy it (not sen).
• Relative and Interrogative Pronouns ('joka', 'mikä'): Because they act like nouns or demonstratives in this context, they take the endingless nominative form (joka, mikä) instead of the standard genitive-accusative (jonka, minkä) when they are the object of a necessive sentence.
  Examples: Kirja, joka minun täytyy ostaa - The book that I must buy. / Mikä minun pitää valita? - What must I choose?
• Negation: As with all negative sentences in Finnish, if the necessive sentence is negative, the object must be in the partitive case.
  Example: Minun ei tarvitse ostaa autoa - I don't need to buy a car.

This rule applies to several 'necessive' verbs and constructions, including täytyy, pitää, on pakko, kannattaa, and the on -tava structure. Distinguish two constructions with tarvita: Minä tarvitsen auton (I need a car) has a personal, conjugated verb and a noun object. The necessive construction has a genitive subject and infinitive: Minun ei tarvitse ostaa autoa (I do not need to buy a car). It also occurs in questions, Tarvitseeko minun ostaa auto?, and in affirmative restricted-necessity contexts, Minun tarvitsee vain ostaa auto (all I need to do is buy a car). Unrestricted affirmative minun tarvitsee is especially associated with speech; minun täytyy/pitää is the neutral learner choice for ordinary obligation. The noun-versus-infinitive construction, not negation alone, determines which object pattern applies. Finally, if the infinitive verb is a partitive verb (like auttaa), the object remains in the partitive regardless of the necessity.

Partitive remains possible in affirmative clauses: Minun täytyy juoda vettä (I must drink water), Meidän pitää ostaa omenoita (we need to buy some apples), and Sinun täytyy lukea kirjaa hetki (you must read the book for a while). Mass/indefinite quantity, unbounded action, and a verb’s lexical case requirement still favor partitive. Apply the endingless rule only after deciding that the object is total.