Actually, try common English phrase: maybe "fydyw" = "could" or "would" or "every".
But possibly it's a (or +21) for the whole thing. Let's test that on tht (if tht is cipher): t(20)-5=15→o, h(8)-5=3→c, t(20)-5=15→o → "oco" — no.
But maybe it's a Caesar shift of 21 (or -5)? Let's check fydyw shift back 5: f(6)-5=1→a, y(25)-5=20→t, d(4)-5=25→y, y→t, w(23)-5=18→r → atytr — no.
Try could : c(3), o(15), u(21), l(12), d(4). Differences: c→f=+3, o→y=+10, u→d=-17, l→y=+13, d→w=+19 — no. Download- fydyw tjss ly lhm mharm mn tht qb a...
Wait — "Download-" is plaintext. Then "fydyw" — maybe it's "email"? No.
Check fydyw on QWERTY row: f → left is d, y → left is t, d → left is s, y → t, w → left is q → dtstq — no.
Shift backward by 5: f (6th letter) -5 = a (1st), y (25th) -5 = t (20th), d (4th) -5 = y (25th? that's y, but 4-5 = -1 → wrap: 26-1=25 → y), y (25th) -5 = t, w (23rd) -5 = r → at ytr — no. Actually, try common English phrase: maybe "fydyw" =
But "qb" would then be "is" if q→i (shift -8) and b→s (shift +17) — no.
This looks like a cipher or encoded message. Let me break it down.
But maybe it's "a t y t r" — no. Could be "at ytr" — nonsense. But maybe it's a Caesar shift of 21 (or -5)
Let's look at the last part: "... mn tht qb a..." — "mn" could be "in" or "on". If m→i: m(13) to i(9) = -4; n(14) to n(14) if word "in"? Then "tht" would decode with same -4: t(20)-4=16→p, h(8)-4=4→d, t→p → "pdp" — not "that".
Given the constraints, the most common trick for such puzzles: (each letter replaced by the key to its left on QWERTY).