Lenition and fortition are names of sound changes usually occurring over many hundreds of years and show up when comparing words from different dialects or different eras (Irish is one of the few language where there are many context changes that are lenition). So there may not be many good examples residing entirely in modern English.
The word 'maternal', created directly from the Latin word 'mater', English for 'mother', has a 't' pronounced as strongly and aspirated as possible. That 't' is a vestige of the more ancient Proto-Indo-European, from which both Latin and English come.
Then 'mother', the 't' has changed, weakened, lenited, to a fricative.
Then the French version of 'mother', 'mère', has lost the original 't' entirely, it has weakened away to nothing a all.
Going in the other direction, strengthening or fortition, the 'th' in 'the' is soft or weak. But in some dialects or in young kids just learning, 'the' is pronounced 'duh', as in 'Duh Bears'. That is, going from 'the' to 'duh' is the process of fortition.
Lenition is more common among world languages than fortition (the 'lazy speaking' theory of the cause of sound change). It follows a number of paths of weakening, usually from
unvoiced stop to voiced stop to affricate to fricative to h/glottal stop to nothing
stop to palatalized consonant to affricate and so on.
or some similar weakening trend.
For English speakers in the US and UK, the culturally most obvious expression of lenition is not in English itself but in the comparison of Latin, Spanish, and French. A 'k' sound in Latin became (almost always) at the beginning of a word a hard 'g', cattus -> gato. In French, cattus -> *kyat -> tchat (Old French) -> chat (Fr) pronounced as 'shah'. Lots of Spansish words that start with 'g' came from a Latin 'k' sound (written as 'c') and lots of French words starting with 'ch' came from the same place.
Don't get me started on Irish. What a mess.