mestinya

Mestinya is a modal adverb used to describe what is ideal, expected, or logical. While its root mesti implies a strict requirement, the addition of the suffix -nya functions as a fixed adverbial marker (similar to biasanya or sebenarnya) that turns the word into a comment on the entire clause.

Key characteristics include:

• Logical Expectation: Used when you expect something to be true based on facts (e.g., 'It should be raining by now').
• Counterfactual Nuance: It is frequently used to express regret or a missed expectation in the past (e.g., 'You should have told me').
• Placement: It usually appears before the verb or predicate, but can be placed at the start of a sentence for emphasis.
• Negation: To say 'should not,' the negation usually follows the modal: mestinya tidak....

In terms of register, mestinya and seharusnya are the standard, neutral choices for daily speech and writing. While they are largely interchangeable, semestinya is another standard form; it is not restricted to literature or highly formal situations. Note that while harus is a synonym for the root mesti, it is a distinct word.

Expectation And Requirement
Dia mestinya sudah sampai sekarang. (He/she should have arrived by now) presents an expectation. Dia harus sudah sampai sekarang. (He/she is required to have arrived by now) can state a requirement or deadline. Both are grammatical; choose the modal for the intended meaning.