Ελέγχω την αλυσίδα, το πεντάλ και τη σέλα.

The article changes because each noun has its own gender, and here they are all in the accusative singular because they are the direct objects of Ελέγχω.

• η αλυσίδα → την αλυσίδα
  feminine singular
• το πεντάλ → το πεντάλ
  neuter singular
• η σέλα → τη σέλα / την σέλα
  feminine singular

So the different articles are not random: they match the gender and case of each noun.

Both τη σέλα and την σέλα are possible, but τη σέλα is the more natural form here in normal speech and writing.

Greek often drops the final -ν from την when the next word begins with certain sounds. Since σέλα starts with σ, it is very common to say and write:

• τη σέλα

You may still see την σέλα, especially in more careful or formal writing, but τη σέλα is completely standard.

They are in the accusative case.

The verb Ελέγχω means I check / I inspect, and it takes a direct object. In Greek, direct objects usually appear in the accusative.

So:

• την αλυσίδα
• το πεντάλ
• τη σέλα

are all accusative singular forms.

Yes. Ελέγχω is the 1st person singular present tense, so it means:

• I check
• I am checking

Like in English, the exact translation depends on context. Greek present tense can often cover both a simple present and a present continuous meaning.

The dictionary form is ελέγχω itself.

Greek verbs are normally listed in the 1st person singular present form, unlike English, where dictionaries usually give to check. So when you look up a Greek verb, you usually find a form like:

• ελέγχω = I check

From that, you learn the verb stem and how it conjugates.

Because this is the normal Greek way to list things, just like in English:

• την αλυσίδα, το πεντάλ και τη σέλα

This works exactly like:

• the chain, the pedal, and the saddle

You do not need και between every item. One και before the final item is enough.

In natural Greek, it is much more normal to repeat the article here:

• την αλυσίδα, το πεντάλ και τη σέλα

Leaving out the articles would sound less natural in most ordinary contexts. Greek uses the definite article more often than English does, especially with specific objects.

So if you are talking about the chain, the pedal, and the saddle of a bike you are checking, Greek strongly prefers the articles.

Yes, το πεντάλ is singular and means the pedal.

This sentence is talking about one pedal as an item being checked. If you wanted the pedals, you would use the plural:

• τα πεντάλ

The word πεντάλ has the same form in singular and plural, but the article tells you which one it is:

• το πεντάλ = the pedal
• τα πεντάλ = the pedals

Their genders are:

• αλυσίδα — feminine
• πεντάλ — neuter
• σέλα — feminine

You can usually tell from the dictionary article:

• η αλυσίδα
• το πεντάλ
• η σέλα

Gender matters because it affects the form of the article and sometimes other words in the sentence.

A rough pronunciation guide is:

• Ελέγχω → eh-LENG-kho
• την αλυσίδα → teen ah-lee-SEE-tha
• το πεντάλ → to pen-DAL
• και τη σέλα → ke tee SE-la

A few useful notes:

• γχ in Ελέγχω is pronounced roughly like ngh / nkh
• αι in και sounds like e
• δ in modern Greek sounds like the th in this
• θ would be the th in think, but that letter does not appear here

Yes, the punctuation here works much like in English. The sentence lists three things:

• την αλυσίδα
• το πεντάλ
• τη σέλα

Greek commonly separates listed items with commas and uses και before the last one, just like English.

One small difference from English in general is that Greek often does not use the Oxford comma before και, so:

• την αλυσίδα, το πεντάλ και τη σέλα

is the normal style.

Yes, depending on context, ελέγχω can have a range of meanings such as:

• I check
• I inspect
• I examine
• I test
• sometimes I control or I monitor

In this sentence, with bicycle parts like chain, pedal, and saddle, the most natural meaning is I check / inspect.

Because the sentence is referring to each part as a category or individual item on a bicycle:

• the chain
• the pedal
• the saddle

Greek often uses the singular this way when talking about parts of one object or one machine. If the speaker wanted to emphasize multiple bicycles or multiple parts, the plural could be used, but the singular is the most natural choice here.