真子がこの手紙をかいたはずはない。

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)

letter

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to scratch; to perspire; to shovel; to paddle

expectation that something took place, will take place or was in some state; it should be so; bound to be; expected to be; must be; nock (of a bow); nock (of an arrow); nock-shaped grip (between thumb and forefinger); wooden frame on the tip of the mast of a Japanese ship that prevents the hawser from falling out

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

not; emphatic suffix