As an illustration of this we may take the future tense.
The Russian verb has commonly a simple and a frequentative future.

The latter is always regularly formed by means of an auxiliary with the infinitive, as in English, but the former is constructed in a variety of ways, for which no rule can be given, so that the simple future of each individual verb must be learned by a pure effort of memory.

In many verbs it is formed by prefixing a preposition, but it is impossible to determine by rule which preposition should be used.

Thus idu (I go) becomes poidu; pishu (I write) becomes napishu; pyu (I drink) becomes vuipyu, and so on.
Closely akin to the difficulties of pronunciation is the difficulty of accentuating the proper syllable.

In this respect Russian is like Greek; you can rarely tell a priori on what syllable the accent falls.