Is this sentence grammatically

To Chun Shan: There are many instances where an infinitive noun phrase led by a bare infinitive is used as the object, e.g. "All I do is sit all day", so "all you do is let me down" is correct. "Let" is a bare infinitive here.

For the sentence to be grammatically correct, it has to be " All you have/need
to do is to let me down. In 1965 in London, I was aghast upon hearing for the
very first time " ....she's got a ticket to ride...., but she don't care." in ' Ticket
to Ride'

2011-05-01 23:29:07 補充：
of The Beatles. Never take it for granted that lyrics must be all grammatically
correct. However, you are right in writing ' to let me down' which serves the purpose as a noun phrase in the sentence.

2011-05-04 12:46:40 補充：
O, my foot again ! It is a basic grammatical knowlege that ' A Phrase doing the work of a Noun is called a Noun Phrase.' , be it a Verb Infinite or simply
Infinitive, with or without adjuncts, may be used as a noun; the latter thus
becomes a Noun Phrase.

2011-05-04 12:56:12 補充：
Equally, a Participial Phrase depending on its use, can either be a Noun
Phrase, e.g., ' Swimming/Jogging with friends' is a good exercise., or an
Adjective Phrase. e.g., I saw a girl' carrying a basket of flowers'. Don't tell
me, " These are Participial Phrases ! ".

chun shan, here is my suggested answer:
"All you did is to down me."

2011-05-03 18:14:57 補充：
"to let me down" is not a noun phrase. It is an infinitive phrase. e.g. (1) To fool the benighted is one of your personalities. (2) To ape other artists is one of his strengths. (3) Your job will be to look after the children.