ذهبت إلى المركز اليوم، لكنه لم يكن مفتوحا في الساعة التاسعة.

It can mean either one in normal unvoweled Arabic writing.

• ذَهَبْتُ = I went
• ذَهَبَتْ = she went

Arabic usually leaves out short vowels, so context tells you which meaning is intended. If your translation says I went, then the intended form here is ذَهَبْتُ.

Because ذهب إلى is the standard MSA pattern for to go to.

So:

• ذهب إلى المركز = he went to the center
• ذهبت إلى المركز = I/she went to the center

In other words, إلى is the preposition that expresses movement toward a destination here.

المركز means the center, while مركز means a center or just center in an indefinite sense.

In this sentence, the speaker is talking about a specific center, so Arabic uses the definite article الـ. Also, after the preposition إلى, the noun is grammatically in the genitive case, so in full vocalization it would be إلى المركزِ.

اليوم means today, and it functions as a time expression.

It tells you when the action happened: went today. Arabic often places time expressions fairly flexibly, so you could also say something like ذهبت اليوم إلى المركز with the same basic meaning.

In fully inflected MSA, it is often analyzed as an adverbial accusative: اليومَ.

It means but it or but he, depending on what it refers to.

Here it refers to المركز, which is a masculine singular noun in Arabic. So even though English uses it for things, Arabic uses a normal masculine pronoun: ـه.

So:

• لكن = but
• ـه = him/it (masculine singular attached pronoun)

Together: لكنه = but it.

Because Arabic nouns have grammatical gender, even when they are not people.

مركز is grammatically masculine, so later references to it use masculine agreement:

• لكنه = but it
• مفتوحًا also matches a masculine singular subject/reference

This is normal in Arabic. English mainly uses natural gender, but Arabic uses grammatical gender much more widely.

لم is a negation particle that usually gives a past meaning and requires the following verb to be in the jussive form.

The verb here is from كان / يكون (to be). In the jussive, يكون becomes يكن.

So:

• يكون = he is / he will be
• لم يكن = he/it was not

Even though English uses a past tense form, Arabic expresses this with لم + imperfect/jussive.

This is the word مفتوحًا in fully vocalized spelling, meaning open.

It is not a main verb here; it is an adjective/passive participle describing the state of the center. With كان, the predicate is in the accusative, so you get:

• كان مفتوحًا = it was open
• لم يكن مفتوحًا = it was not open

In everyday Arabic writing, the final case ending is usually not written, so you often see مفتوحا instead of the fully marked مفتوحًا.

This is the normal MSA way to express clock time.

Literally, it is something like in the ninth hour, but idiomatically it means at nine o'clock. So the whole phrase works just like an English time expression.

Also, after في, the phrase is in the genitive, so with full endings it would be في الساعةِ التاسعةِ.

Because ساعة is a feminine noun, and the ordinal adjective must agree with it.

So:

• الساعة التاسعة = the ninth hour / nine o'clock
• not الساعة التاسع

Arabic adjectives agree with the nouns they describe in gender, number, definiteness, and case.

Because س and ت are sun letters.

When الـ comes before a sun letter, the l sound of the article is not pronounced, and the next consonant is doubled in pronunciation:

• الساعة is pronounced roughly as-sāʿa
• التاسعة is pronounced roughly at-tāsiʿa

In writing, the ل is still there, but in pronunciation it assimilates.

If the meaning is I went to the center today, but it was not open at nine o'clock, a fully vocalized version is:

ذَهَبْتُ إِلَى الْمَرْكَزِ الْيَوْمَ، لَكِنَّهُ لَمْ يَكُنْ مَفْتُوحًا فِي السَّاعَةِ التَّاسِعَةِ.

If the subject were she instead of I, the first word would be ذَهَبَتْ instead of ذَهَبْتُ.