Synthetic vs. analytical languages.
All languages are either synthetic or analytical. A synthetic languages shows grammatical relationships through inflectional changes in the words, whereas an analytical language represents the grammatical relationships through word order. English was once synthetic, but over time has become an analytical language.
Masculine, Feminine and Neuter.
In Old English, gender was applicable to words as well as their case. case being either nominative (the subject), genetive (possessor), accusative (the direct object) or dative (the indirect object). To understand this concept it is important to know that when english uses gender, they are not implying that everything is either a boy, girl or neither, but simply refering to class, which is essential for conjugation in synthetic languages. English no longer uses grammatical genders, and only uses biological gender in the case of he, she and it. Other languages however still use grammatical gender, for example French.
-S as an Ending.
By Medieval times the difficult system of masuline, feminine and neuter was beginning to diminish, resulting partially from the success of -s as a plural ending. Determining word endings no longer totally relied on case and gender, but was instead normalising as -s became a popular choice for new words. This was the beginning of English shifting from a synthetic language to an analytical one.
Strong and Weak Verbs.
Verb conjugations like fly - flew and run - ran were much more common in Old English than they are in the Modern English we speak today. They were known as 'strong verbs', while verbs that hold the -ed ending were known as 'weak verbs'. The past tense ending -ed has now become the standard and come to replace the other, 'stronger' endings (bring - brang, ring - rung), just as the plural ending -s came to replace other, more difficult choices. This is why 'strong verbs' are known today as 'irregular verbs', put simply, they are no longer the norm and are few and far between when compared with the 'weak' verbs.
The Process of Analogy.
The standardising of both -s and -ed endings are great examples of the process of analogy. The process of analogy occurs when a particular ending or rule becomes so popular that it becomes the 'normal' choice and all new words will follow the same process when conjugated. For example, if you post a tweet on twitter, you have tweeted something. notice the -ed ending when used in the past tense? this is the process of analogy.