Be aware that this pronunciation is made in the back of your throat...some natives pronounce the "ch" with the teeth, but the fact of using the throat makes it easy to compare... when pronouncing "-ach", your throat and mouth has to open wider than when pronouncing "-ich", making it sound less rough.. hope it helps
German: Pronunciation of CH (English subtitles)/Aussprache von CH (englische Untertitel)
other pronunciation videos https://www.youtube.com/playlist?list=PL8345BD873EEE18F4
It may help you to think it einfach as meaning "simple" (as an adjective) and "simply" (as an adverb).
Du bist einfach wunderbar! = You are simply wonderful! / You are just wonderful!
Du musst einfach die Tür öffnen. = You simply have to open the door. / You just have to open the door.
"simply/simple" is the connector between the meanings "just" and "easy".
This course generally sticks to "it = es // das = that" and keeps those two separate.
So if you want to get marked correct, it's best to use "it" if and only if the German sentence has "es" (and vice versa), and "das" if and only if the German sentence has "that" (and vice versa).
Essentially, leicht means literally "light(weight)" (the opposite of schwer "heavy") and metaphorically "easy" (the opposite of schwer "difficult").
einfach means "simple" (the opposite of "complicated, complex").
Since simple, uncomplicated things are often easy, einfach is often used for "easy" as well, a bit like how someone who's good at maths might say that "this question is simple" when they mean that it's easy for them to solve, not because it's simple in the sense of "not complex, not complicated, not made up of many individual components".
So in actual use, the meanings shade towards each other a bit, but the basic meanings are leicht = light / easy, einfach = simple.
'Es ist einfach' means 'It is easy'. Whereas 'Das ist einfach' means 'THAT is easy.' Es- It Das- The/That. In Dualingo, it means 'The' generally when it used just before the noun. Das is used in such cases for NEUTRAL nouns.
And when it is not next to noun, then it is used as 'That'.
I hope this helps. :-)
Careful: ch makes two different sounds in German, depending on whether there is a front vowel (e i ö ü) or a back vowel (a o u) before it. (The two sounds are sometimes referred to as der Ach-Laut and der Ich-Laut.)
ch after a back vowel is [x] (a voiceless velar fricative like the "ch" in Scottish "loch"); ch after a front vowel is [ç] (a voiceless palatal fricative similar to the sound that hy- makes in words such as "huge, human" for some English speakers).
-ig at the end of a word sounds like -ich in standard German; thus, lustig and glücklich have the same consonant sound at the end -- which is different from the one at einfach, however.