Many of the exercises on this page are early attempts to create web-based learning material and as such are quite rough. You might still find some of it useful.
A student asked me today why the infinitive ‘to’ can be dropped after ‘help’. Here’s the short answer:
The infinitive ‘to’ is usually dropped when the infinitive form is used in conjunction with a specific set of verbs - these include “feel”, “hear”, “help”, “let”, “make”, “see”, and “watch”. Examples include: