As Krashen suggests in his Natural Order Hypothesis, language acquisition takes place in some sort of natural order. Some certain structures of languages are acquired in order regardless of the input and exposure. According to the sequence of Natural Order Hypothesis, acquisition starts with progressive –ing and plural –s. Then, past irregular and possessive –s are mastered both in L1 and L2 acquisition. As it can be seen from the table below.
That is to say , when the sequence of structures’ acquisition is considered, it is quite normal case that third person singular –s is omitted by language learners because it’s acquired later in the acquisition order. The third person suffix “-s” is the last of this set of morphemes to be mastered by L2 learners of English. It takes time to language learners acquire the third person –s and use it automatically despite the quite amount of exposure and practice as well.