Ispanağı çok haşlarsan rengi solar, bu yüzden marulu ve ıspanağı ayrı tutuyorum.

Because it has taken the definite direct object ending.

• Base word: ıspanak = spinach
• With accusative: ıspanağı = the spinach

Two things happen:

1. The accusative ending is added: -(y)ı / -(y)i / -(y)u / -(y)ü
2. The final k often changes to ğ before a vowel in Turkish

So:

• ıspanak → ıspanağı

This is a very common sound change in Turkish.

In both places, the nouns are specific direct objects, so Turkish uses the accusative case.

• Ispanağı çok haşlarsan... = If you boil the spinach too much...
• marulu ve ıspanağı ayrı tutuyorum = I keep the lettuce and the spinach separate

In Turkish, a direct object is often marked when it is definite/specific.

Compare:

• ıspanak haşlamak = to boil spinach, in general
• ıspanağı haşlamak = to boil the spinach, a specific spinach

Haşlarsan is built with the aorist + conditional:

• haşla- = boil
• haşlar = boils / if one boils
• haşlarsan = if you boil

This structure is very common for general conditions and timeless truths:

• çok haşlarsan = if you boil it too much

Using haşlıyorsan would sound more like if you are boiling / if you happen to be boiling, which is more tied to a specific ongoing situation.
Here, the sentence expresses a general fact, so haşlarsan fits well.

Here çok means too much or for too long, not many.

So:

• Ispanağı çok haşlarsan... = If you boil the spinach too much / too long...

In Turkish, çok can modify verbs as an adverb:

• çok konuşmak = to talk a lot
• çok beklemek = to wait too long / a lot
• çok haşlamak = to boil too much

Rengi means its color or the color of it.

It comes from:

• renk = color
• rengi = its color

This is a possessive form. Turkish often uses this where English would say its color.

So:

• rengi solar = its color fades

There is an implied connection to ıspanak:

• ıspanağın rengi = the spinach’s color

In the sentence, Turkish does not need to repeat the full phrase, so rengi is enough.

This is another common consonant change.

Some Turkish words ending in k change that k to g or ğ when a vowel-initial ending is added.

So:

• renk
  • -i → rengi

This is just something learners get used to seeing in many words.

Other similar patterns:

• çocuk → çocuğu
• yemek → yemeği
• renk → rengi

Solar is the aorist form of solmak, which means to fade or lose color.

• solmak = to fade
• solar = fades / will fade / tends to fade

Here it expresses a general result:

• rengi solar = its color fades

So the first clause is basically a general truth:

• If you boil spinach too much, its color fades.

Because Turkish often leaves things understood from context.

In English, we say:

• its color fades

In Turkish, once ıspanak has already been mentioned, rengi already carries the idea of its color, so no separate word for it is needed.

This is very normal in Turkish.

Because the verb endings already show the subject, and Turkish does not need explicit subject pronouns unless there is emphasis.

• haşlarsan = if you boil
• tutuyorum = I am keeping / I keep

So the sentence moves naturally from a general you in the first clause to I in the second:

• If you boil spinach too much, its color fades, so I keep lettuce and spinach separate.

This kind of shift is perfectly normal.

Bu yüzden means for this reason, because of this, or simply so / therefore.

• bu = this
• yüzden = because of / due to

Together:

• bu yüzden = therefore / that’s why

In this sentence, it connects the result of the first clause to the speaker’s action in the second:

• ..., bu yüzden ... = ..., so ...

Because both nouns are separate definite direct objects joined by ve.

• marulu = the lettuce
• ıspanağı = the spinach

So:

• marulu ve ıspanağı = the lettuce and the spinach

In Turkish, when two definite objects are coordinated, each one is often marked individually.

Literally, it means I am keeping [them] separate.

• ayrı = separate, apart
• tutmak = to hold, keep
• tutuyorum = I am keeping / I keep

So ayrı tutmak is a natural Turkish expression meaning:

• to keep apart
• to keep separate

It is more idiomatic here than translating word-for-word as I hold separate.

Because Turkish -yor forms can express both:

• an action happening now, and
• a habitual/present practice

So ayrı tutuyorum can mean:

• I’m keeping them separate
• I keep them separate

In this sentence, it sounds like a regular practice based on the reason given before it.

Yes. Turkish word order is flexible, although this version is very natural.

Current sentence:

• Ispanağı çok haşlarsan rengi solar, bu yüzden marulu ve ıspanağı ayrı tutuyorum.

You could also say:

• Bu yüzden marulu ve ıspanağı ayrı tutuyorum.
• Marulu ve ıspanağı bu yüzden ayrı tutuyorum.

The basic meaning stays the same, but emphasis changes a little.

• Putting bu yüzden earlier can emphasize the reason link.
• Putting ayrı near the verb highlights the idea of separation.

Usually yes, or at least keeping things apart in some practical sense.

It can be used for:

• foods
• objects
• people
• categories

Examples:

• Beyazları renklilerden ayrı tut. = Keep the whites separate from the colored clothes.
• Bu dosyaları ayrı tutuyorum. = I keep these files separate.

So in your sentence, it sounds like the speaker keeps lettuce and spinach apart, probably in preparation, storage, or cooking.

Yes, it is understandable and natural overall.

A learner should especially notice these useful patterns:

• X-ı çok V-arsan... = If you V X too much...
• rengi solar = its color fades
• bu yüzden = therefore / so
• X ve Y ayrı tutuyorum = I keep X and Y separate

So it is a good example of:

• accusative case
• conditional -sa/-se
• possessive forms like rengi
• practical everyday vocabulary