Kontakt z klientem bywa trudny.

Bywa means something like can be, is sometimes, or tends to be at times. It suggests that this is not always true, but it happens from time to time.

So:

• Kontakt z klientem jest trudny = Contact with the client is difficult
• Kontakt z klientem bywa trudny = Contact with the client can be difficult / is sometimes difficult

This makes bywa softer and less absolute than jest.

Because the preposition z in the sense of with requires the instrumental case in Polish.

The noun klient changes like this:

• nominative: klient
• instrumental: klientem

So z klientem means with a client / with the client.

Because trudny agrees with kontakt, which is a masculine singular noun.

In Polish, adjectives must match the noun in:

• gender
• number
• case

Here:

• kontakt = masculine singular
• so the adjective is trudny

If the noun were feminine, you would get trudna.
If it were neuter, trudne.
If plural, trudni or trudne, depending on the noun.

Yes. Kontakt z klientem is a very natural Polish expression. It can mean:

• direct contact with a client
• dealing with a client
• client interaction

In business or customer-service contexts, kontakt z klientem is extremely common.

Yes, but the meaning changes slightly.

• Kontakt z klientem jest trudny. = Contact with the client is difficult.
  This sounds more direct and definite.

• Kontakt z klientem bywa trudny. = Contact with the client can be difficult / is sometimes difficult.
  This sounds more nuanced and less absolute.

So bywa is often used when the speaker wants to generalize without sounding too strong.

That word order is completely natural in Polish. The sentence structure is:

• Kontakt z klientem = the subject/topic
• bywa trudny = what is being said about it

Polish word order is more flexible than English word order, but the version here is the most neutral and natural.

Yes, but it may sound more marked or stylistically different.

For example:

• Kontakt z klientem bywa trudny. = neutral, standard
• Trudny bywa kontakt z klientem. = emphasizes trudny
• Bywa trudny kontakt z klientem. = possible, but less natural in ordinary speech

So the original sentence is the safest version for learners.

Kontakt is in the nominative singular, because it is the grammatical subject of the sentence.

The structure is:

• Kontakt — subject, nominative
• z klientem — prepositional phrase, with klientem in the instrumental
• bywa trudny — predicate

Yes. In Polish, klient can often mean either client or customer, depending on context.

For example:

• in a shop: customer
• in a business/service relationship: client

So in this sentence, either translation may be possible.

Yes, absolutely. Klientami is the instrumental plural of klient.

• Kontakt z klientem = contact with a client
• Kontakt z klientami = contact with clients

The rest of the sentence can stay the same:

• Kontakt z klientami bywa trudny. = Contact with clients can be difficult.

Yes. Bywa comes from bywać, which is related to być (to be).

A useful way to think of it is:

• być = to be
• bywać = to be from time to time / to happen to be / to occur sometimes

That is why bywa often adds the idea of repeated or occasional situations.

A rough guide is:

Kontakt z klientem bywa trudny
KON-takt z klien-tem BI-va TROOD-nih

A few useful pronunciation notes:

• y in bywa and trudny is not the English ee sound
• w in Polish sounds like English v
• trudny ends with a clear -ny, not an English-style reduced ending

If you want to sound natural, stress is normally on the second-to-last syllable of each word:

• kontakt
• z klientem
• bywa
• trudny