Learn English – Correct usage of ‘on’, ‘at’ and ‘in’

There isn't going to be a quick way to match up propositions. There are some general rules that fit a majority of the cases but the exceptions abound.

Unfortunately, changing prepositions around will have drastic effects on its meaning. Instead of trying to catch and explain all cases, I have decided to simply use your examples and explain what they mean:

I worked on a project

"On" is typical here and will be correct most of the time. Working on a project implies doing work related to the project and is similar to working on a house or working on a letter. You are performing an action (working) on an object (the project).

The tricky side of "on" comes with phrases like, "I worked on a farm." Working "on a farm" means doing work while located on a farm instead of working on the object of a farm.

A good test is to try replacing "worked on" with "completed": "I completed a project" or "I completed a house" sound fine while "I completed a farm" drastically changes its meaning.

I worked in a project

Working "in" something is more akin to being located inside of the object. "I worked in a building." A "project" can certainly contain things and so you can technically work "in" a project.

When I was at the university I studied math

Here, "at" is mostly referring to your physical location. You did work while being located at the university. This generally refers to students enrolled in the university but also applies to people who were just visiting or possibly working there in a way unrelated to learning (such as the janitor.)

When I was in the university I studied math

Again, "in" here implies being "inside of" or "contained by" the university. To say, "I am in university" generally means to be enrolled in one of the university's programs. Unfortunately, it can also mean being physically inside of the university buildings... so I am not helping much.

The most common case is to say, "When I was at..." but there is a mix of things happening here. People will ask you "Where do you go to school?" The importance seems to fall on the physical location but this usage does not cover strange exceptions such as enrolling in classes over the internet where you never move from your chair but are considered "at" a particular university.

In any case, I hope this helped somewhat.

This is correct:

I will tell your consultant.

The verb tell is transitive and takes a direct object that indicates the person that will be spoken to. It is incorrect to add the preposition to.

Edit: Actually, it's a little more complicated than that. Let me explain.

The verb tell is actually ditransitive, meaning that it can take both a direct object and an indirect object:

Let me tell you a story.

Here the bolded you is the "indirect object", and it indicates the person that will hear the story. The italicized a story is the "direct object", and it indicates what will be told to the person. When tell is used ditransitively, you can optionally take the indirect object and move it to a prepositional phrase with to:

Let me tell a story to you.

(Almost all ditransitive verbs in English allow you to move the indirect object to a prepositional phrase with to.)

However, tell can also be used with just a single argument object, and in that case you never add the preposition to.

Let me tell you.

*Let me tell to you. [Wrong!]

This is somewhat unusual, since the object of tell in this case can be either the indirect object or the direct object. Most ditransitive verbs don't work this way:

Let me give you some money. [Ditransitive -- OK]

Let me give some money. [Direct object only -- OK]

*Let me give you. [Indirect object only -- Wrong!]

?Let me give to you. [Indirect object with to -- questionable.]