Countries in high poverty bear the brunt of plastic pollution./ suffer the most from plastic pollution. How can the society help in solving this problem is always a headache. Earlier this year, a pilot project of an environment rehabilitation campaign was set out to clean up Philippines' Manila Bay beach. Locals can collect the trash and then trade it for croptcurrency provieded by some Banks. This pilot project did make a great success. Over 2 months, Locals, mostly fishermen, collected three tonnes of trash from Manila Bay beach. And 2 months later, another project was underway. Around 5,000 volunteers came to visit Manila and collected over 45 tonnes of garbage. Manila Bay beach is unrecognizably clean compared with 4 months ago. 聼起來自然嗎?

*Countries in high poverty bear the brunt of plastic pollution./ suffer the most from plastic pollution.

- both are correct

*How can the society help in solving this problem is always a headache.

-How can society help in solving this problem.

-It is always a headache for society to solve this problem.

*Earlier this year, a pilot project of an environment rehabilitation campaign was set out to clean up Philippines' Manila Bay beach.

-However, earlier this year....

- Because the previous sentence you mentioned how difficult it is to solve this problem use however. In speech you can say: But, earlier this year. However, in writing you don’t want to start a sentence with: and, but, or or.

*Locals can collect the trash and then trade it for croptcurrency provieded by some Banks.

-perfect

*This pilot project did make a great success.

- This pilot project was a great success.
- This pilot project continues to be successful.

*Over 2 months, Locals, mostly fishermen, collected three tonnes of trash from Manila Bay beach.

-over two months means more than two months.

- For two months locals and mostly fishermen collected three tonnes of trash from Manila Bay beach.
- In just/only two months, locals and mostly fishermen collected three tonnes of trash from Manila Bay beach.

- In english when you put two nouns side by side, it is the same noun.

-This drink, milk tea, is delicious. (sorry but I forgot the name of this grammar structure)

*And 2 months later, another project was underway.
- and 2 months later indicates after 4 months.(2 months of cleaning beach, then two more months passed, now we are starting month 5.

-And 2 months later, another project was underway. (your sentence is perfect)
- Two months after the success of the first project, another project began.


*Around 5,000 volunteers came to visit Manila and collected over 45 tonnes of garbage. Manila Bay beach is unrecognizably clean compared with 4 months ago.

-perfect
- I see you meant to say 4 months(great job!!), and you were correct. I will leave it just for practice.

If I made any mistakes or you still have any questions. Let me know 吧。