Do I have to file Form 26AS with my Indian return?
No. It is not a form you file at all. It is a statement the department assembles against your Indian identifier out of what other people have reported: tax deducted by payers, tax collected at source, and tax you paid directly. Nobody submits it on your behalf and there is nothing to sign. What is expected of you is the other half of the exercise, which is reading it before you file and reconciling what it shows against your own records. The return carries your figures. The statement is what those figures will be measured against.
My certificate shows tax but Form 26AS does not, who fixes it?
The deductor, and only the deductor. Credit is given for what appears in the statement, not for what the certificate in your hand says, so a certificate with no matching entry is a document you cannot spend. The entry is created by the payer's own periodic reporting, which means a missing or mis-keyed entry has to be corrected at that end. In practice it is a matter of identifying which payment and which period is absent, showing the payer its own certificate, and asking for the correction to be filed. Doing that before you file is far easier than doing it after a credit has been cut.
I live in Canada, does Form 26AS still matter for my Indian income?
It matters more, not less. A non-resident's Indian income is usually income somebody else has already deducted from, whether that is interest on deposits, rent, or the proceeds of a sale, and the statement is where you find out whether the deduction was actually reported against you. It is also the document that tells you what the department believes about your Indian year while you are not in the country to notice. Anyone claiming relief elsewhere for Indian tax will be asked to show what was deducted and what became of it, and this is the record that answers that.
Who actually puts the entries into Form 26AS?
Third parties do. Payers who deduct from what they pay you, collectors who take tax at the point of a transaction, and the banking channel that records tax you pay yourself. Each reports periodically against the identifier it was given, and the statement is the sum of those reports. That is why the identifier you hand a payer matters so much. An entry filed against a wrong or superseded identifier does not attach to you and does not appear. It has not vanished, but it is not yours until the payer corrects its report.
Can I claim credit for tax that is missing from Form 26AS?
You can enter it, but expect it to be disallowed on processing, because the credit is matched against the statement rather than against your certificate. There are two sensible orders of work. The better one is to find the gap before filing, get the payer to correct its report, and file once the entry is visible. The other is to file on the correct figures and be ready to evidence the missing deduction while the payer's correction goes through. Either way the certificate alone does not settle it, and treating the statement as a formality is how a credit is lost.
When should I check Form 26AS before filing my return?
Early enough that a payer still has time to correct something. Entries appear after each reporting period, so a year's statement only becomes complete some way into the following year, and the last deductions to arrive are usually the ones with a problem in them. Checking it at the moment you are ready to file leaves no room to do anything about a gap except file and argue. Checking it once the reporting for the final period has gone through, and again immediately before filing, is the sequence that keeps the choice in your hands.
What happens if the two countries disagree about which of them can tax me?
The treaty has a procedure for exactly that. You apply to the competent authority in your residence country, which takes the case up with its counterpart, and the two negotiate a position that removes the double taxation. Some treaties add binding arbitration if they cannot agree. It is slow and it runs on documents, so the practical work is preserving the record and filing protective claims while the clock runs. See our treaty work.
Do I get credit for all of the foreign tax I paid?
Only up to your own country's tax on that same income, and only for tax you were legally obliged to pay. Two consequences follow. Living somewhere that taxes you more heavily than your residence country does leaves an excess that becomes a carryover rather than a refund. And withholding suffered above the treaty rate is not creditable — the route back to that money is a refund claim in the country that took it. See claiming the credit.