You have already used the passive with is / are and was / were. The reason for choosing it does not change when the time changes: put the receiver of the action first when the doer is unknown, obvious, or not important, and add by + the doer only when you want to name them. The only thing that changes from here on is the the verb be.
Why you choose it never changes
Active: Someone stole my bike. → Passive: My bike was stolen. (we don't know who)
Active: They clean the office. → Passive: The office is cleaned. (the doer doesn't matter)
Naming the doer: The office is cleaned by a company.
Not "Active: The team built the bridge. (focus on the team)" — say "Passive: The bridge was built. (focus on the bridge)". Same event — the passive just puts the receiver first and can drop the doer.
Try it: You don't know who broke the window. Which sentence is the natural choice? (The window was broken. / Broke the window by someone. / The window broke somebody.) Answer: The window was broken. — When the doer is unknown, the passive puts the receiver (the window) first and leaves the doer out: The window was broken. You only add "by …" when you actually want to name the doer.
The past participle never changes
You already build the past participle the same way as before. The point of this lesson is what stays still: whatever the time, the past participle is exactly the same. Only the verb be moves.
Now: They paint the wall. → The wall is painted.
Past: They built the house. → The house was built.
Already finished: They have written the letter. → The letter has been written.
One past participle, three times — only is / was / has been changed.
Try it: Complete the passive: "The letter was ___ yesterday." (verb: write) (written / writed / wrote) Answer: written — "Write" is irregular, so its past participle is written — not "writed." "Wrote" is the past of the base verb; the passive needs the past participle: The letter was written.
The passive shape stays the same
The passive is always a form of be + the past participle. To change the time, you change only the verb be. The past participle never changes. For now (a regular thing), use is / are + the past participle.
They wash the car every week. → The car is washed every week.
They check the tickets at the door. → The tickets are checked at the door.
Someone cleans the office daily. → The office is cleaned daily.
Try it: They wash the car every week. Turn this into a regular-thing passive. (Leave out the word you don't need.) Answer: The car is washed every week. — A regular thing uses is + the past participle: The car is washed every week. "Wash" is the base verb, but the passive needs the past participle washed, so leave it out.
Change the verb be for the past
For the past, swap the verb be to was / were. The past participle is exactly the same as before.
Someone washed the car yesterday. → The car was washed yesterday.
They built the houses last year. → The houses were built last year.
A guard locked the gate. → The gate was locked.
Try it: Someone washed the car yesterday. Turn this into a past passive. (Leave out the word you don't need.) Answer: The car was washed yesterday. — For the past, the verb be is was: The car was washed yesterday. The past participle washed doesn't change; only the verb be does, so "is" (the now form) is left out.
Try it: A shop follows the same daily routine: someone cleans the floor before the doors open. Which sentence fits a regular thing happening now? (The floor is cleaned every morning. / The floor was cleaned yesterday morning.) Answer: The floor is cleaned every morning. — A regular thing happening now uses the verb be is: the floor is cleaned every morning. "Was cleaned" points to one finished time in the past (yesterday), so the time signal is what tells them apart.
Use will be for the future
For the future, the verb be becomes will be (will + be). The past participle still never changes. Keep the be — will on its own is not enough, because the passive always needs a form of be.
They will wash the car tomorrow. → The car will be washed tomorrow.
A team will build the bridge next year. → The bridge will be built next year.
Someone will send the letter soon. → The letter will be sent soon.
Try it: They will wash the car tomorrow. Turn this into a future passive. (Leave out the word you don't need.) Answer: The car will be washed tomorrow. — For the future, the verb be is will be: The car will be washed tomorrow. "Will" on its own would drop the be (the very thing the passive needs), so use will be and leave "will" out.
Negatives and questions
To make the passive negative, put not after the verb be: is not washed, was not washed, will not be washed (short forms: isn't / wasn't / won't be). To make a question, move the verb be in front of the subject: Is the car washed? Was the car washed? Will the car be washed? The past participle never moves.
Negative now: The car isn't washed on Sundays.
Negative past: The gate wasn't locked last night.
Question now: Is the office cleaned every day?
Question future: Will the letter be sent tomorrow?
Not "The car not is washed. / Washed the car is?" — say "The car isn't washed. / Is the car washed?". Not goes right after the verb be; a question moves the verb be to the front.
Try it: Make it negative (past passive): "The gate ___ last night." (verb: lock) (wasn't locked / didn't locked / wasn't lock) Answer: wasn't locked — Put not after the verb be (was → wasn't) and keep the past participle: the gate wasn't locked. "Didn't" belongs to active verbs, and "lock" is the base verb, not the past participle.
Try it: The car is washed every week. Make this a yes/no question. (Leave out the word you don't need.) Answer: Is the car washed every week? — For a passive question, move the verb be (is) in front of the subject: Is the car washed every week? "Does" belongs to active questions, not the passive, so leave it out.