A substitute is something that's the thing you wanted, but actually another thing. It can be pretty confusing but as long as you keep thinking of the thing you want/need and get something similar, that thing shouldn't be too wrong.
If you want to make sure no-one gives you something you don't need instead of the thing you wanted you have to check the thing he gives you and make sure, really sure, really really sure, that it's like the thing you want/need.
If you don't do this you may get something that's not at all like the thing you wanted but is in fact something completely different.
In this case it is not a substitute.
Substitute Explained in Song by The Shandelles
- You think we look pretty good together
- You think my arse is made of leather
- But I'm a substitute for a bowl of soup
- I smell pretty nice but it's perfumed poop
- The simple things you see are all complicated
- I look pretty fresh, but I'm just back-dated, yeah
- Substitute your thighs for crack
- I can eat right through your cheese and mac
- I look all white, but my dad was black
- My fine bicycle is all out of whack
- I was born with a singing fish in my mouth
- The family moved north but grandpa and I went south
- And now you dare to poke me in the eye
- Big fat tears are what you made me cry
- I've a genuine problem with this sty
- You won't medicate it, you just pass it by, pass it by
- Substitute the us for we
- Substitute my beer for pee
- Substitute me for my pa
- At last I get into your bra...