Going Shopping

I made this exercise a couple of days ago about shopping. 


The last time I went shopping was last month. I bought a bag, a white blouse and a black slacks. The bag costs 150 pesos. The white blouse costs 280 pesos and the black slacks costs 190 pesos.

Revised:

The last time I went shopping was last month. I bought a bag, a white blouse and black slacks. The bag cost 150 pesos. The white blouse cost 280 pesos and the black slacks cost 190 pesos.


In this exercise, I was confused with the use of "cost"

My question: "I thought since I am speaking of a singular subject so I need to write costs. "

Answer: 

• Ana, you are correct that whether a subject is singular or plural can affect whether to use "cost" or "costs."
  
  For plural subjects, you will use "cost" in both the present tense and the past tense.  For example, "Last year, sandwiches cost $4.00 at this cafe.  Can you believe sandwiches now cost $5.00?"
  
  For singular subjects you will use "costs" in the present tense and "cost" in the past tense.  "Last year, a milkshake cost $3.00.  Now a milkshake costs $3.25."
  
  Subject/verb agreement can sometimes seem confusing.   For example, "a carton of eggs" is singular (because the subject is actually the carton), so we would say "A carton of eggs costs $2.00."  However, changing the way you word the sentence can change the verb: "Eggs cost $2.00 per carton."  When deciding which verb form to use, ignore any prepositional phrases that come in between the subject and the verb.   You can think of it as  "A basket (of strawberries) costs = a basket costs."  Therefore, you would say, "A basket of strawberries costs $1.50."  "Strawberries cost $1.50 per basket."  "A box of Cheerios costs less than most other breakfast cereals."  "Cheerios cost less than most other breakfast cereals."