speak /spiːk/ - the base form of the verb, spelled S-P-E-A-K.
Can you speak more slowly? The ea spells /iː/, the long vowel in see, not /e/, so avoid writing spek. Each remaining sound matches its letter: /s/, /p/, /k.
Natural uses
Can we speak after class? The final /k/ is spelled k, not ck, even though you hear the same sound at the end of back.
They often speak in the evening. Here ea spells /iː/, the long vowel in see, so the written letters stretch past their usual /e/ value.
The 'ea' spelling trap
In speak, the letters ea spell /iː/, the long vowel you hear in see, not /e/. That mismatch is the trap: writing spek reflects the sound /e/ but misses the actual spelling. Trust /spiːk/ over letter intuition here.