The \ a decision to change the government has been taken.
1) which article should I use?
Earlier this year, with the help of the Sydney Harbour Federation Trust, we reached a decision to expand our study and start putting rubbish back into particular areas of the harbour.
2) The above sentence is copied from an IELTS book. I wonder why "a decision" is written instead of "the decision" as it is specific ("expanding our study and start putting rubbish back into particular areas of the harbour.")
We use the whenever the noun is specific, even if it is first time mentioned, don't we? For example: "a decision about my live" is general, the decision in this case can be anything so we use "a". My example confusing me because it is a specific action and a specific decision, there is only one probability which is putting the rubbish over there. I still don't know how this decision in this context can be considered general.