Here’s another one from VIM:

Have you ever wondered about special characters and how to get the into VIM?

Copy-and-paste is one solution, yes. But there is a whole list of characters that can be easily entered directly.

Use the command digraph, to get an overview:

:digraph
:digraph                                                                                                                                               
NU ^@  10    SH ^A   1    SX ^B   2    EX ^C   3    ET ^D   4    EQ ^E   5    AK ^F  6i
BL ^G   7    BS ^H   8    HT ^I   9    LF ^@  10    VT ^K  11    FF ^L  12    CR ^M  13
SO ^N  14    SI ^O  15    DL ^P  16    D1 ^Q  17    D2 ^R  18    D3 ^S  19    D4 ^T  20
...
tk ﺙ  1579   g+ ﺝ  1580   hk ﺡ  1581   x+ ﺥ  1582   d+ ﺩ  1583   dk ﺫ  1584   r+ ﺭ  1585
z+ ﺯ  1586   s+ ﺱ  1587   sn ﺵ  1588   c+ ﺹ  1589   dd ﺽ  1590   tj ﻁ  1591   zH ﻅ  1592
e+ ﻉ  1593   i+ ﻍ  1594   ++ ـ  1600   f+ ﻑ  1601   q+ ﻕ  1602   k+ ﻙ  1603   l+ ﻝ  1604 
...

This probably breaks in HTML, so best to try it youself.

There are at least two ways enter them. CTRL+k should always be available.

So, while being in insert-mode, press CTRL+k and the first two keys mentioned in the overview.

CTRL+k tk == ث