Clauses of Manner explain how the action in the main clause is done or describe the state of the subject. They are introduced by 'As, As if, and As though'.

1. 'As' is used with the factual manner. For example:

    1. John finished the assignment on time as he was told.
    2. Anne will call the principal to talk about her son as she discussed with         her husband.
    3. Things go smoothly as we planned.

2. 'As if' and 'As though' are used with hypothetical statements. For example:

    
   1. John looks as if he is tired. (certain)
   2. John looks as if he were tired. (less certain)
   3. John looks as though he stayed up all night. (uncertain about the        fact/less certain)   
  4. John completed the job as if he wished to be fired.
  5. Anne ate as if she hadn’t eaten for days.

Note that the verb form in the clause of manner introduced by 'as if' and 'as though' are:

1. present indicative: V1 (+s/es)

    This form is used when the writer is more certain about what he says in     the clause of manner.

2. past subjunctive: V2 (with 'be' as 'were')

    This form is used when the writer is less certain about what he says in     the clause of manner.

Moreover, the perfect aspect of a verb can also be expressed as in Example 5.

More about Tenses