Tenancy Issues

Your case is partially cleared to me. There must be a LOI or similar kind of a mutual trust or an agreement between you and the actual tenant (who has an agreement with the owner of the house under his name PUB bill coming on a monthly basis).

If you would say that you have had a word based on trust to stay in that house for a period of time then you r free once that time is passed or if you and the actual tenant are agreed to break the trust and allow you to go prior to your agreed terms then you too can leave happily.

If you would say that you have absolutely no obligation with the actual tenant and you stayed there for a short time then you are free to go. But, I would suggest to check with the actual tenant who has 2yrs lease with the owner and if he didn't have an issue then there is nothing to be worried. But, it surprised to me that you intend to leave that place by 01 Sep'16 and you have this query in your mind in last week of Aug'16, which means you may not have discussed about your plan with the actual tenant.

If I understand this correctly, somebody else rented the apartment until October and you had an agreement (verbal, which is also a contract, just more difficult to prove - you need witnesses!) with that tenant that you take over their remaining lease. Thus you are subtenant. Now you want to end your lease in September.
(Please correct if this is not true!)
Now, whether you can get out earlier depends entirely on your contract with the original tenant - if there was nothing agreed on, you cannot terminate earlier.
Even if you terminate earlier, the original tenant will have to pay the rent until October.
In any case, you cannot use the deposit to offset the last rent - that is illegal, as the deposit is for covering any damagey etc. after you move out.

I recommend you talk to the landlord (and/or main tenant) to find a mutually agreeable solution - especially if you want to continue staying and renting in Singapore!

I have actually informed the main tenant around 1st of July that we'll be moving out by Sept which she agreed upon.We even managed to get someone to take over our place.However, after few weeks she and the other tenants changed their mind and decided not to renew their contract.Furthermore,they are insisting that we should all stay until the contract expires despite informing them that we are leaving by Sept 3.My question now is should we still pay for the last month whether we stay or not?Thanks

This problem arises due to there is no paper or agreement proof. If you have had an agreement then based on that you could have done legally but in this case you want to terminate the contract earlier than the expiry ( in Oct), then the main tenant has every rights not to return your deposit.

I would suggest if the main tentant is adamant and not willing to return your deposit when you leave that place by 1st week of Sep'16, then decide carefully whether you are willing to lose your deposit or to stay Sep month as any how from Oct this agreement would be void?