Identify and correct the errors in the following sentences: 

 a) Neither the doctor nor the nurse are available. 

b) The dog fell along the river. 

c) Apples are not sweet as mangoes. 

d) Helen is junior than me. 

The errors are as follows: 

[a] Because the words 'doctor' and 'nurse' are singular, the singular 'is' should be used rather than the plural 'are'.

[b] The right preposition here is 'into,' not 'along.' The word 'along' makes the phrase nonsensical.

To indicate direction, time, place, location, spatial relationships, or to introduce an object, a preposition is a word or set of words used before a noun, pronoun, or noun phrase. Prepositions include phrases like "in," "at," "on," "of," and "to." English prepositions are quite idiomatic.

[c] The right conjunction is the correlative 'as...as' rather than a single 'as.'

[d] The comparative 'junior' is preceded by 'to' rather than 'than.'