Оптик сравнивает оправу и линзы.

Here оптик means an optician / the optician, so it is a person.

A related word is оптика, which can mean an optics shop or optics as a field. In this sentence, because we have оправу and линзы, оптик clearly means the person working with glasses.

Russian has no articles, so there is no separate word for a/an or the.

That means Оптик сравнивает оправу и линзы could mean:

• The optician is comparing the frame and the lenses
• An optician compares a frame and lenses

Context tells you which one is meant.

Because of case.

• оптик is in the nominative case, which is the normal case for the subject.
• оправу and линзы are objects of the verb.

So the sentence structure is:

• оптик = the doer
• сравнивает = compares / is comparing
• оправу и линзы = the things being compared

Because оправа is a feminine noun, and here it is a direct object, so it goes into the accusative singular.

For this type of feminine noun:

• nominative: оправа
• accusative: оправу

So:

• оправа = frame
• оправу = frame (as the object of the verb)

It is changing by case, but for this noun the accusative plural looks the same as the nominative plural.

• singular: линза
• plural nominative: линзы
• plural accusative (inanimate): линзы

Since линза is an inanimate noun, the plural accusative is the same as the plural nominative.

Линзы means lenses, so it is plural because there is more than one lens.

That does not mean the whole pair of glasses is being called линзы. If you want to say glasses as a complete object, the usual word is очки.

So:

• оправа = frame
• линзы = lenses
• очки = glasses

Сравнивает is:

• 3rd person singular
• present tense
• imperfective
• from the infinitive сравнивать

So it means:

• he/she compares
• he/she is comparing

Here it agrees with оптик, which is singular.

It can mean either one.

Russian present tense often covers both:

• a general / habitual meaning: compares
• an action happening now meaning: is comparing

So without more context, сравнивает can be understood both ways.

Сравнивает is imperfective.

That suggests:

• an ongoing action
• a repeated action
• a general process

A common perfective partner is сравнить.

Examples:

• сравнивает = is comparing / compares
• сравнит = will compare
• сравнил = compared, finished comparing

A useful point: Russian perfective verbs do not have a true present tense. So if you want a present-time meaning like is comparing, you use the imperfective.

That is a very good question.

Very often Russian does use:

• сравнивать что-то с чем-то
• for example: сравнивать оправу с линзами

That pattern is often the clearest way to say compare one thing with another.

The sentence Оптик сравнивает оправу и линзы is understandable as a learner sentence meaning that the optician is comparing those two items. But in many real-life contexts, сравнивает оправу с линзами may sound more explicit and natural.

Yes. Russian word order is more flexible than English because the cases already show the grammatical roles.

The neutral order here is:

• Оптик сравнивает оправу и линзы.

But other orders are possible for emphasis, for example:

• Оправу и линзы сравнивает оптик.

That sounds more marked and emphasizes who is doing the comparing.

So yes, you can change the order, but the default textbook order is the most neutral one.