Learn Hardcore Turkish: Ben masayı güzelce temizliyorum.

You can drop it. The verb ending in temizliyorum already shows first person singular. Use Ben only for emphasis or contrast (e.g., “I (as opposed to someone else) am cleaning it”): Masayı güzelce temizliyorum is perfectly natural.

Why is it masayı and not masa?

Because the table is definite/specific. Turkish marks definite direct objects with the accusative case.

Definite object: masayı (I’m cleaning the table we both know about)
Indefinite object: masa or more commonly bir masa (I’m cleaning a/some table)

How do I know which accusative ending to use: -ı/-i/-u/-ü?

Use 4-way vowel harmony based on the last vowel of the noun:

Last vowel a or ı → -ı (e.g., masa → masayı)
Last vowel e or i → -i (e.g., kedi → kediyi)
Last vowel o or u → -u (e.g., okul → okulu)
Last vowel ö or ü → -ü (e.g., göl → gölü)

What is the y doing in masayı?

It’s a buffer consonant used to separate two vowels: masa + ı would create vowel clash, so it becomes masa + y + ı → masayı. Turkish often inserts y (and sometimes n/s) at morpheme boundaries to keep pronunciation smooth.

How is temizliyorum built morphologically?

Root adjective: temiz (clean)
Verb-forming suffix: -le- → temizle- (to clean)
Present continuous: -iyor/-ıyor/-uyor/-üyor → here -iyor
1st person singular: -um/-üm/-ım/-im → here -um (harmonizing with the preceding o in yor)
So: temizle + iyor + um → temizliyorum (with a regular vowel change explained next).

Why isn’t it temizleiyorum? Where did the extra vowel go?

When a verb stem ends in -a/-e and you add -yor, that vowel typically shifts to -ı/-i:

anla- + -yor → anlıyor
bekle- + -yor → bekliyor
temizle- + -yor → temizliyor
Then you add the personal ending: temizliyor-um → temizliyorum.

What’s the difference between temizliyorum and temizlerim?

temizliyorum: present continuous (I am cleaning right now; also used for near-future with a time word).
temizlerim: simple present/habitual (I clean [it] regularly/typically; statements of fact or routine).

How do I make it negative?

Insert the negative -me/-ma before -yor:
temizle-me-yor-um → temizlemiyorum
Full sentence: Ben masayı güzelce temizlemiyorum.

How do I ask a yes/no question?

Use the question particle mi (which harmonizes as mi/mı/mu/mü) as a separate word:
Masayı güzelce temizliyor muyum?
Word order around mi is strict: it follows the verb complex.

Where should I put güzelce?

Default and most natural here: just before the verb: Masayı güzelce temizliyorum.
You can move elements for emphasis (focus tends to be right before the verb):

Güzelce masayı temizliyorum. (focus on the object: it’s the table I’m cleaning nicely)
Masayı güzelce temizliyorum. (focus on manner: nicely/properly is highlighted)
Placing güzelce after the verb is usually awkward.

Can I say güzel instead of güzelce?

Yes, in everyday speech many adjectives can function adverbially: Masayı güzel temizliyorum is common.
Nuance: güzelce is explicitly adverbial (“nicely/properly”) and can sound a bit more deliberate; güzel is very colloquial and broad (“well/nicely”).

What does the suffix -ce/-ca in güzelce do?

It often turns adjectives/nouns into adverbs of manner (roughly “-ly”): güzel → güzelce (“nicely”).
It has other roles too (e.g., language names: Türkçe), but here it’s adverbial. Harmony chooses -ce because güzel has a front vowel (e).

How would I say “my table” here?

Use possessive + accusative: masa + m + ı → masamı.
Full sentence: Ben masamı güzelce temizliyorum.
Order of suffixes: possession first, then case.

Can I omit the object or replace it with a pronoun?

Yes. If context makes it clear, you can drop it: Güzelce temizliyorum.
Or use a pronoun: Onu güzelce temizliyorum (“I’m cleaning it nicely”), where onu is the accusative of o (“it/that”).

What’s the nuance difference between temizlemek and silmek?

temizlemek: to clean (general, any method).
silmek: to wipe/wipe down (with a cloth/tissue).
If you specifically mean wiping the table, Masayı güzelce siliyorum is more precise. If you mean washing/scrubbing/overall cleaning, temizlemek fits.

Can this sentence also refer to a planned future action?

With a time expression, yes: Yarın masayı güzelce temizliyorum can mean “I’m cleaning the table tomorrow (it’s arranged).” For a neutral future, you can also use the future tense: temizleyeceğim.

How do I pronounce the dotless ı in masayı and temizliyorum?

Turkish ı (dotless i) is a central, unrounded vowel, somewhat like the second vowel in English “roses” for many speakers. It’s not the same as English “uh,” but that’s a closer approximation than “ee.”
Examples: masayı (ma-sa-yı), temizliyorum (te-miz-lı-yo-rum).