Principal Parts

The Principal Parts of a verb is the collection of six forms of the verb that serve as a basis for building any other Tense-Mood-Voice-Person-Number form of this verb. Below is the chart with the Principal Parts of λύω. 

In this chart, bullets are used to separate different morphemes (i.e. building blocks) of each form. λυ is the root of the verb. The morphemes appearing before the root are called prefixes (here ε or λε). The morpheme on the end of the word is called the ending (here ω, α, μαι, or ην). The morpheme that appears between the root and the ending is called the suffix (here σ, κ, or θ). The part of the word without an ending is called a stem. So, for example, λυ is the stem of the first Principal Part, λυσ is the stem of the second Principal Part, and so on. 

Under each Principal Part, you can see which Tense-Voice combinations are built on the stem of this principal part. The same stem is then used for all moods, persons, and numbers. 

You can read about the formation of principal parts in the following places:

Each Principal Part, together with the ending, also represents the 1st Person Singular form of a certain Tense-Mood-Voice combination.