The rule that catches most people out: money you take out of a TFSA does not free up room until January 1 of the following year. Check before you re-contribute.
Enter the room you had on January 1 before any contributions this year.
Most registered accounts work the way you would expect: take money out, and you can put it back. A TFSA does not work that way within the same calendar year. The amount you withdraw is added back to your contribution room on January 1 of the following year, not immediately.
So if you have $2,000 of room, withdraw $10,000 in March, and try to put the $10,000 back in June, you have over-contributed by $8,000 — even though the money was yours and it came out of the same account a few months earlier. The penalty is 1% per month on the highest excess amount, for as long as it stays in the account.
On January 1 of the year following the withdrawal. A withdrawal in January and a withdrawal the following December both restore room on the same date — the next January 1. That makes late-year withdrawals slightly more attractive if you plan to re-contribute, because the wait is shorter.
1% per month on your highest excess amount for each month it stays in the account. There is no grace period and no cap, so a large accidental over-contribution gets expensive quickly. The CRA does have a process to waive or cancel the tax in genuine, reasonable-error cases, but you have to apply for it.
No. Only the dollar amount you actually withdraw is added back. If you contributed $10,000, the account fell to $6,000, and you withdraw the $6,000, only $6,000 returns to your room. The $4,000 lost is gone permanently from a contribution-room perspective.
No. TFSA withdrawals are not taxable income, are not reported on your tax return, and do not affect income-tested benefits like the GIS or the Canada Child Benefit. This is a key difference from RRSP or RRIF withdrawals.