If you have had a chance to check out Estonian dictionary, you may have noticed that with some words there are some additional forms provided. Also if you have taken a closer look you would notice root variances in these forms, i.e. couple letters in the word vary.

From English, I would recall changes such as loaf-loaves, or sing-sang-sung etc. Such phenonemon is called gradation and it involves letter changes in different forms of a word.

In Estonian, about 4/5 of words change in various forms making the fact of change almost a constant. There is no reason to worry about it! You'll get used to it by listening and reading Estonian as you will learn to notice the patterns of gradation.

Now couple examples of gradation:

A double letter may be reduced to one and you could hear the sound quantity to change:     

                                                                                kk ~ k    kokk koka (chef)

                                                                                pp ~ p    lipp lipu (flag)  

                                                                                tt ~ t      pott poti (pot)

G, B, D affected by gradation assimilates with the neighboring consonant:   

                                                                               mm ~ mb    hammas hamba (tooth)

                                                                               nn ~ nd    rand ranna (beach)

                                                                               rd ~ rr     kord korra (order)

                                                                               ld ~ ll     sild silla (bridge)

Double vowel, vowel combination, or consonant combination varies in quantity – a variance between 2nd and 3rd quantity:                                                                                laulu – laulu  (song)

                                                                               kulmu – kulmu (eyebrow)

                                                                               kooli - kooli    (school)