(someone) has had to (do something)

Use this for something that you had to do in the past, and you still have to do it. In other words, use this to talk about an obligation that started in the past and has continued until the present. For example, if you lost your job and haven't found a new one yet, you can say:

I've had to cut back on my expenses and rely on my friends and family for help.

walk up and down the stairs

When complaining about stairs, people often use the phrase "walk up and down". This sounds tiring.

If you're not talking about how tiring the stairs are, you can use the phrase "take the stairs" instead:

The elevator was broken so I took the stairs.

a flight of stairs

"Stairs" can't easily be counted, but here are ways that you can specifically count them:

• One whole set of stairs in a building is called a "staircase". Most buildings only have one staircase, but some large buildings might have 2 or 3 separate staircases.
• One "flight of stairs" connects two floors. For example, between the ground floor and the 2nd floor is one flight of stairs; between the 2nd and 3rd floor is another flight, and so on. There can be several flights of stairs in one staircase.
• Sometimes there will be a "landing" halfway up a flight of stairs. This is a wider area where the stairs turn around and go in another direction.
• A staircase or a flight of stairs is made up of lots of "steps". You usually walk up one step at a time.