[Ashley]

The phrase "a batch of" can be followed by countable and uncountable nouns
but why in Word it must be followed by singular verb even through the subject
is plural? Can anyone help me in explaining this?

By grammar of Subject and Verb Agreement and Principle of Proximity:

A batch of letters are ready. (Should be correct)

The reason is that the subject "A batch of letters" actually is plural and
should be followed by a verb in plural form. I have also tried €œa number of€�
and €œa group of€� and they are OK.
A number of STBs are ready. (Correct in Word)
A group of STBs are ready. (Correct in Word)

I have checked in dictionary and grammar book:
http://www.merriam-webster.com/dictionary/batch
http://www.merriam-webster.com/thesaurus/batch

Dictionary:
1 : the quantity baked at one time : baking
2 a : the quantity of material prepared or required for one operation;
specifically : a mixture of raw materials ready for fusion into glass b : the
quantity produced at one operation c : a group of jobs (as programs) that are
submitted for processing on a computer and whose results are obtained at a
later time batch processing €” compare time-sharing
3 : a quantity (as of persons or things) considered as a group

Thesaurus:

Meaning: 1 a number of things considered as a unit a batch of essays to
correct €” see group 1
2 a usually small number of persons considered as a unit send in the next
batch of applicants €” see group 2


Subject and Verb Agreement
Basic Rule. The basic rule states that a singular subject takes a singular
verb, while a plural subject takes a plural verb.

http://www.grammarbook.com/grammar/subjectVerbAgree.asp