I'm not really adding anything to what Ainurakne already said, but I thought I'd post a couple of other explanations of the same grammar point. Basically Estonian has täissihitis
/"total object" and osasihitis
/"partial object" rather than "accusative." Sometimes the total object (or the partial/total object set) is called accusative, although it isn't a separate case but rather a combination of cases. Also, don't pay any attention to this site which says that accusative is the partitive case. It's misrepresentations or oversimplifications like that which make me avoid using the word "accusative" entirely because there is too much confusion and inconsistency surrounding that term with regards to Estonian grammar.
Honestly, the total/partial object system is one of the more difficult aspects of Estonian grammar for speakers of other languages. Don't let the idea that "Estonian doesn't have an accusative case" fool you into thinking that makes it simpler than languages that do. from Estonian Institute's overview of the Estonian language:
Despite the large number of cases, the Estonian language lacks the ordinary object case, the accusative, which is common among the Indo-European languages. The direct object in Estonian is expressed by the nominative, genitive or partitive, in the singular, and by only the nominative or the partitive in the plural. Using the genitive object in the singular and the nominative object in the plural, marks the totality and finiteness of the action directed at that object. The usage of the partitive case expresses the partiality or unfinished nature of the action.from Estonian for Beginners (Oser/Salasoo):
As a general rule, the total object must be used when all of the following three conditions are met:
1. the sentence is a positive statement
2. the action of the verb has been completed
3. the action of the verb embraces the whole, total object.
In cases where all three of the above conditions are not met, the partial object is used. The partial object is always in the partitive case, answering the questions mida? or keda?
The total object in the singular is either in the genitive case, answer the questions mille? or kelle? or, for positive commands, in the nominative case, answering the quetsions mis? or kes? The total object in the plural is always in the nominative case, answering the questions mis? or kes?from Estonian Textbook (Tuldava):
The direct object (called sihitis in Estonian) can be considered either as a "total" object or a "partial" object. The total object (täissihitis) may also be called the definite, complete, or whole object. It is used in sentences were all three of the following conditions are met:
1) The sentence is affirmative.
2) The action of the verb leads to completion.
3) The object is effected in its entirety.
In an imperative sentence (where a command is given), the total object is in the nominative case. In a declarative sentence (where a statement of fact is given), the total object is in the genitive case if it is in the singular and in the nominative case if it is in the plural.
Total object examples:Võta raamat! (nom. sing.) = Take the book!
Võta raamatud! (nom. pl.) = Take the books!
Ma võtsin raamatu. (gen. sing.) = I took the book.
Ma võtsin raamatud. (nom. pl.) = I took the books.
Loe raamat läbi! (nom. sing.) = Read the book [all the way through]!
Ma lugesin raamatu läbi. (gen. sing.) = I read the book [all the way through].
Ma ostan raamatu. (gen. sing.) = I will buy the book.
Partial object examples:Ära võta raamatut! (part. sing.) = Don't take the book!
Õpilane loeb raamatut. (part. sing.) = The student is [engaged in] reading a book.
Loe raamatut! (part. sing.) = Read [some of] the book! (but not necessarily to the end)
Ma lugesin raamatut. (part. sing.) = I was reading the book. (unfinished)
Ma ostan raamatut. (part. sing.) = I am [engaged in] buying the book.