Well, that's why I didn't say the experience would make him a good teacher, but that having experience creates additional value. I've always found that teachers who have real world experience can supplement the material with real-world experiences to improve the overall learning experience.

For example, the material might treat all subjects the same, but an instructor with real-world experience can flag the important stuff. There are always a few pieces where you just have to understand the concept, but there are great tools available so you won't be doing the heavy lifting here... but this thing over here is an area where people frequently make mistakes and it breaks things, so pay close attention to it.

I think that technically that's not true. Although 'taking' is a present participle and 'shown' is a past participle, both verbal groups do not indicate the grammatical tense. In that particular sentence only the main verb ('is' as in There is no...) is marked for tense (ie. the present tense).

The expression "There's no harm in..." should be followed by the present participle (indicated by the -ing suffix). The only difference is that 'taking' is in the active voice, and the grammatically correct form, 'being shown,' would be in the passive voice. The fact that 'shown' is a past participle has no influence on the grammatical tense. Please compare:
Yesterday I was shown a new house (the past tense)
I do not need to be shown what to do when I get intimate with my girlfriend (the present tense)
Tomorrow I'll be shown a new house (Well, when it comes to English verbs there's no future tense, the future aspect is indicated by means of auxiliary verbs, eg. will)

?

It seems like the first would be more correct, since "had" refers to the past and "was" fits with that. The question marks in the sentences look like they shouldn't be there, either.

?

I'd say the first one is more correct, because you're listing more than one thing that you "find unnecessary", and "is" is usually only used with one item. (<- see what I did there? ).

and what is used with multiple items with what logic?

"are"

Anisha, what is not necessary?

I am and was, confused.
The two questions I posted here are both w.r.t, is and was.
One with tense problem and another with plural/singular
problem.
Now, I got confused as to which one was Mr. Code responding
to, with ""is" is usually only used with one item".

"perpendicular angles" and "perfection in placement",
OR I couldn’t get your question?

I think MrCode was referring to the second pair of sentences.

Ok, than you're supposed to use 'are'. With the existing punctuation your sentence is saying that "mixing colours and patterns" is not necessary.

Your idea with this picture (I guess) is to show the mixing "colours" and "patterns", for which (IMO) "perpendicular angles" and "perfection in placement", are NOT necessary.

To be honest, I don't quite understand what you're trying to say (that's probably my fault - I haven't had coffee yet).
Is it?
The idea behind this picture is to show a process of mixing colours and patterns.
or
The idea behind this picture is to show that mixing colours and patterns, ......., is not necessary (ie. mixing is not necessary)

Now "for which perpendicular angles and perfection in placement is/are not necessary":
Does it refer to:
a) patterns
b) colours and patterns
c) the whole process of mixing a) and b)?

This is turning interesting now, the answer is C.

Unfortunately, I am not able to read the sentence the way
you are reading it (perhaps since I am used to writing it
this way).

The only way, I put the commas is, by repeating the sentence
in my mind, and thinking, after which set of words I need to
take a pause, before speaking further. (That's what I have
done to this sentence also)
Pardon my dumbness.

Ok, so if I understand it correctly, it's supposed to be as follows:

The idea behind this picture is to show the process of mixing 'colours' and 'patterns', for which 'perpendicular angle' and 'perfection in placement' are not necessary. (Note there's no comma after 'placement')

Having said that, bear in mind I'm not a native speaker so other guys will probably correct me.

I won't. It looks like you nailed it to me.

Wow. You picked a tough one for your example. It has more wrong with it than just the commas, but here's my stab at reformatting it: