Traditionally, linguists think of morphemes as constant sound-meanings. That is to say, when a standard noun is to be made plural (exceptional rules exist for non-standard nouns, of course), the inflectional morpheme -s at the end of it changes the word to indicate pluralism. Cat/Cats. Beer/Beers. Along the same token, when a standard verb is to exist in the past tense (again, exceptions do exist for non-standard verbs), the inflectional morpheme -ed changes the word to denote its tense. Bake/Baked. Raise/Raised. Sometimes, however, our bizarre little English language becomes fuddled in its own upbringing.
Take the following sentences:
(1) I love looking at the sheep in the meadow.
(2) I hit them with a giant baseball bat.
These sentences illustrate the remarkable concept known as the zero-form morpheme--a morpheme that doesn't change the word at all, typically in spelling or pronunciation.
In the examples provided, so long as the sentences are independent from each other (pretend that each sentence was not followed/preceded by the other--in fact, while looking at one, forget that the other two exist) there is no real way to know what the morphology of the italicized word actually is.
In (1), sheep could be singular or plural. If there's a field of sheep outside my house, I can enjoy looking at them. If I have a painting of one sheep in a meadow, I love looking at it. In (2), tomorrow, when the sheep annoy me, I will hit them with a bat. Or, yesterday, when the sheep annoyed me, I hit them with a bat.
Put another way, words that have zero-form morphemes in them require additional information (from context in the sentence or, if unavailable, from surrounding sentences) in order to understand what the semantic meaning is. If the sentence is
(3) The sheep that are in the meadow are pleasant to look at.
the plural form of "sheep" is known from context, as the plural verb form are exists to imply sheep + plural. The second sentence could be rewritten
(4)Yesterday I hit them with my baseball bat.
and the time adverb yesterday implies that the verb is hit + past.
This is all fine and well, but because of the traditional view of morphemes being sound objects, it is suggested that the morpheme hit is marked as both present and past in the lexicon, and the morpheme sheep is marked as both singular and plural, and that there are no zero-forms in English, and the ones that apparently exhibit such qualities are generally borrowed words.
There are a great many languages that exist that utilize zero-form morphology to a great extent, however. It was 2500 years ago, the Hindu grammarian Panini (often thought of as the greatest linguist ever) suggested that some morphemes have no phonological representation. Turkish, Hindu, Korean, and even African American Vernacular English (AAVE, Ebonics) contain words that nicely attach themselves to zero-form morphemes.