Analyze the following scheme, point out the elements of the sentence and the predicative construction. Compare it to the scheme of the same construction with the Infinitive

(Reason 2) Use the correct verb after using an object of a preposition.

The object of a preposition cannot be the subject of a verb.

• 
  Lee's box of magazines are under the stairs. 
  
• 
  Lee's box of magazines is under the stairs. 
  

Don't be fooled by the proximity of the object of the preposition (here, "magazines") to the verb. You must ensure the subject (here, "box") and the verb agree in number.

However, the situation is different with some expressions (e.g., "half of," "a proportion of"). With these, the object of the preposition does influence the verb.

• 
  Half of the cakes are missing. 
  

(Here, "cakes" means that "half" is treated as plural.)

• 
  Half of the cake is missing. 
  

(Here, "cake" means that "half" is treated as singular.)

Read more about subject-verb agreement.