Diagnosta mierzy bieżnik.

Because some Polish masculine nouns, especially job titles and personal nouns, end in -a. Diagnosta is one of them.

So even though it looks feminine at first sight, it is grammatically masculine when it refers to a man:

• ten diagnosta = this diagnostician / inspector
• dobry diagnosta = a good diagnostician / inspector

This is similar to words like mężczyzna (man) or kolega (male friend / colleague), which are also masculine even though they end in -a.

Mierzy is the 3rd person singular present tense form of the verb mierzyć.

So:

• ja mierzę = I measure / I am measuring
• ty mierzysz = you measure
• on / ona mierzy = he / she measures

In this sentence, mierzy agrees with the singular subject diagnosta.

Bieżnik is the direct object, so it is in the accusative case. But for many masculine inanimate nouns in Polish, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: bieżnik
• accusative: bieżnik

That is why the word does not visibly change here.

Yes. When mierzyć means to measure something, the thing being measured is usually in the accusative.

Examples:

• mierzyć ciśnienie = to measure blood pressure
• mierzyć temperaturę = to measure temperature
• mierzyć długość = to measure length
• mierzyć bieżnik = to measure the tread

So the grammar of this sentence is very standard:
subject + verb + direct object

Polish present tense often covers both:

• he measures
• he is measuring

So Diagnosta mierzy bieżnik can mean either:

• The inspector measures the tread (habitually / generally), or
• The inspector is measuring the tread (right now)

The exact meaning depends on context.

Because mierzyć is imperfective, and its present tense can describe an action in progress or a general repeated action.

Its perfective partner is often zmierzyć. But perfective verbs do not have a true present meaning in Polish. A form like zmierzy usually means will measure, not is measuring.

So:

• mierzy = measures / is measuring
• zmierzy = will measure

That is why mierzy is the natural form here.

A rough guide for an English speaker:

• mierzy ≈ MYE-zhy
• bieżnik ≈ BYEZH-neek

A few key sound notes:

• mie- / bie- sound like a soft mye / bye rather than plain me / be
• rz and ż are pronounced like the s in measure or the zh sound in vision
• nik is close to neek, with a short Polish i

You do not pronounce rz as two separate sounds.

Yes. Polish word order is more flexible than English because case endings help show what each word is doing.

The neutral order here is:

• Diagnosta mierzy bieżnik.

But other orders are possible for emphasis, for example:

• Bieżnik mierzy diagnosta.
• Mierzy diagnosta bieżnik.

These are grammatically possible, but they sound more marked and would usually depend on context. For a basic neutral sentence, Diagnosta mierzy bieżnik is the best choice.

Because Polish has no articles. There is no direct equivalent of English a or the in ordinary sentences.

So Diagnosta mierzy bieżnik could mean:

• A diagnostician / inspector measures a tread
• The diagnostician / inspector measures the tread

Context tells you which one is meant.

Traditionally, diagnosta is a masculine profession noun, and in many contexts it refers to a man. For a woman, many speakers use diagnostka, though usage can vary by profession and register.

The important learner point is this:

• diagnosta may end in -a
• but it is not automatically a feminine noun

So you should learn its grammatical behavior, not just judge by the ending.

Not always. Bieżnik can have different meanings depending on context. In technical or automotive contexts, it commonly means tread, especially tire tread. In everyday household contexts, it can also mean a table runner.

So context is very important with this word. In this sentence, because someone is measuring it, the automotive meaning is the natural one.