What is mental phenomenon? Every internal sense organ comes along with corresponding external organ. For example eye comes along with form, ear comes along with sound, nose comes along with smell , mind comes along with mental phenomenon.
I am aware of the meaning of form or sight , sound , smell etc .. but I am not sure what is meant by mental phenomenon? 
When eye and form/sight come together eye consciousness arises.
When nose and smell come to together nose consciousness arises.

In both the statement above it is clear from my experience what is sight and smell. But when we say mind and mental phenomenon come together mind consciousness arises, it is not clear from my experience ,what is the mental phenomenon? Is mental pehnomenon the memory of past or the simulation of future or is it the dream or is it the speech ?
There are different types of tastes ,colors ,odours, touch etc... Are there different types of mental phenomenon also?
Phenomena source is one of the twelve sources. It is apprehended only by mental consciousnesses: In Commentary to the Middle-Length Lamrim, Geshe Jampa Gyatso says:


  The twelve sources are the six object-possessors (the eye source, ear
  source, nose source, tongue source, body source, and mental source)
  and the six objects (form source, sound source, odor source, taste
  source, tangible object source, and phenomena source).


There are two types of mental consciousnesses:


Conceptual consciousnesses
Non-conceptual mental consciousnesses


An example of a conceptual consciousness is a memory-consciousness. Examples of non-conceptual mental consciousnesses are the five clairvoyances, and dream-consciousnesses. The five clairvoyances are: minds apprehending minds in the continuum of other persons, the divine eye, and so forth.

If it is a phenomena source, it cannot be apprehend by a sense consciousness. However, if it is a mental consciousness, it does not necessarily apprehend phenomena source. For example, the divine eye apprehends actual form (i.e. form which is form source).

Furthermore, an example of a mental consciousness apprehending phenomena source is a dream-consciousness. A dream-car is called 'form that is not form source but phenomena source', it is not fully qualified form. In Commentary to Lorig, Geshe Tenzin Tenphel says:


  The blue of a dream is a phenomena source, an object of mental
  consciousness, whereas actual blue is a form source, an object of eye
  consciousness.


An example of a valid conceptual consciousness is a memory-consciousness remembering one's mother. Its appearing object is a mental image, and the actual mother is the object of engagement. A mental image is a phenomena source, and therefore a conceptual consciousness apprehends its object of engagement by way of a phenomena source appearing.


According to Asanga and Vasubandhu's Abhidharma texts, here are five examples of phenomena source:


Dream-objects
Kasinas (such as the water or earth kasina to the perspective of a yogi)
Appearances of hair falling, etc. appearing to an eye that is subject to disease.
The appearance of two moons when there is one moon in actuality.
The appearance of an actual face when we look at the reflection of a face in the mirror.