I had worked (past perfect) and I worked (past simple)

We use the past perfect to talk about a past situation or activity that took place before another past situation or activity, or before a particular time in the past: 

John discovered that Ted had stolen his money. 

First Action: Ted had stolen

Second Action: John discovered

As I introduced myself to her, I realized I had seen her before. 

First Action: I had seen her before. 

Second Action: I realized

We use the past simple rather than the past perfect when we simply talk about a single activity or «event in the past:

For example,

I lent him some money. 

I took a bath late this morning. 

Notice the difference in the meaning of these sentences with the past perfect and past simple:

When I finished cleaning, everyone arrived. (= they arrived after I stopped cleaning)

When I finished cleaning, everyone had arrived.(= they left before I finished laughing)

I got up when the phone rang. (= the phone rang and then I got up)

I had gone to bed when the phone rang. (= I went to bed and then the phone rang)

We use the past perfect when we say what we wanted or hoped (etc.) to do but didn't:

I had wanted to buy the famous cake in that city before I come back home, but they didn’t have available stock. 

Bill had hoped to retire at 60, but they persuaded him to stay on for a few more years. 

Other verbs used like this include expect (to), mean (to), think (about + -ing).

When we use a time expression (e.g. after, as soon as, before, by the time (that), when) to say that one event happened after another, we use either the past simple or past perfect for the event that happened first and the past simple for the event that happened second:

As soon as Ted (had) finished his homework, he went straight to bed. 

When Ted (had) known that she was sick, he visited her. 

But to emphasize that the second event is the result of the first, we prefer the past simple for both:

She became famous after she appeared on a TV program.

When the teacher came in, all the children stood up. 

With already and just (= a very short time before) we use the past perfect, not the past simple:

The film had already begun by the time we got to the cinema.

She had just stepped into her office when the telephone rang.