@Hairy Scot

While you are right that outside of mathematics or engineering, "perpendicular" is supposed to mean "vertical", I would say that in everyday speech and print, it more common to see/hear "vertical" instead of "perpendicular" to describe something that is at right angles to the the horizon/horizontal.

@Hairy Scot
I actually said in preference to meaning 1a not 1b. I agree meaning 1a is related to pendre - pendulum, pendulous, pending

When someone says "I took a chair perpendicular to his" they probably mean that the chairs are oriented such that one is facing (say) north and the other is facing east (from meaning 1b).

I would not necessarily use "perpendicular" in everyday conversation though. I would probably say "at right angles". To describe a 90 degree orientation, sometimes the word "square" is used.
"The front wall should be square with the side wall".

Merriam Webster Online gives the following:

Definition of PERPENDICULAR

1 a : standing at right angles to the plane of the horizon : exactly upright
b : being at right angles to a given line or plane

2: extremely steep : precipitous

3 often capitalized : of or relating to a medieval English Gothic style of architecture in which vertical lines predominate

4: relating to, uniting, or consisting of individuals of dissimilar type or on different levels

I guess the word that means 1a does not get used very often in everyday conversation. People mostly use "upright" or "vertical" or the phrase "standing straight up". Meaning 1b however is very commonly used in geometry and engineering terminology, where we say Line CD is perpendicular to Line AB, irrespective of the inclination of line AB to the horizontal - meaning the angle between the two lines is 90 degrees.

Becky,

There is a difference.
Consider these sentences (ignore the factuality of these examples!) :

1. Children playing basketball at night is bad.
vs
2. Children playing basketball at night are bad.

Both are correct (but, of course, have different meanings). The sentence in the original post by WW is like number 1.

In addition to what AnWulf said, I would like to comment that, yes, it is possible to change words/phrases/clauses to make a particular word choice correct, but that is not what the question was about.

For example, if I present a sentence: " There are eight inches between the two ends of this stick. ", and if my question is (for example) "Is eight inches" correct or is "eight-inches" correct? -- Helpful responses would focus on that, and not just say something like: " Change the sentence to: This is an eight-inch long stick. "
(unless, of course, the entire original sentence does not make sense).

"is" is fine here. "is" goes with "one" - you are referring to one of the things you believed in.

You would use "are" if you were to say:
There are all sorts of things I believed in then which I don't believe in now. Language rules set in stone and unchanging pronunciation are two of those things.

It is like saying: Seven days is a long time to finish this work. vs There are seven days in a week.

AnWulf, in your examples for "is", .... How long is it? It is eight inches long. (It + is)

It is an eight-inch board. ...... "It is" refers to the board not to the length.

Generally, if the context is in the present or recent past then "has" is used and if the context is in the distant past then "had" is used ......

Today Mark got a letter of acceptance from Penn State. He is very happy. He has always wanted to go to college in his home state.

Yesterday Mark got a letter of acceptance from Penn State. He is very happy. He has always wanted to go to college in his home state.

On that day in 1981, Mark got a letter of acceptance from Penn State. He was very happy. He had always wanted to go to college in his home state.

I guess some of the questions we have on this forum are examples of spoken language, which are probably alright anyway if that is how people speak in a particular setting or group and everybody gets the intended meaning.

Yes, Max, I agree your sentence sounds even better.