صديقي يسقي الصبار.

It is made of:

• صديق = friend
• ـي = my

So صديقي means my friend.

This ـي is a possessive suffix attached directly to the noun.

Because صديقي is already definite.

In Arabic, a noun with a possessive ending like ـي (my) is automatically definite. So Arabic says:

• صديقي = my friend

not:

• الصديقي

In other words, Arabic normally does not combine ال and a possessive suffix on the same noun.

يسقي is the imperfect verb, which usually corresponds to the present in sentences like this.

It is:

• 3rd person
• masculine
• singular

So it means he waters or he is watering, depending on context.

The basic past form is سقى = he watered / gave water to.

It is pronounced yasqī.

A fully vowelled version is:

• يَسْقِي

Notes:

• ya- at the beginning marks he
• the س has no vowel here
• the final ـي gives a long ī sound

So it sounds roughly like yas-kee, but with a real q sound for ق, not an English k.

Because Arabic allows this kind of consonant cluster inside a word.

In يَسْقِي:

• سْ has sukūn (no vowel)
• then comes ق

So the word is yasqī, not yasaqī.

English speakers often expect every consonant to have a vowel after it, but Arabic does not work that way.

Because it is definite: the cactus.

Arabic often uses ال not only for a specific thing, but also for a whole class in a general sense. So الصبار can mean:

• the cactus in a specific context, or
• cactus/cacti as a general kind of plant

In a simple learner sentence, the most natural reading is just the cactus.

Even though it is written with ال, it is not pronounced al-s... here.

That is because ص is a sun letter. With sun letters, the ل of ال is not pronounced, and the next consonant is doubled.

So الصبار is pronounced:

• aṣ-ṣabbār
  or, with a full case ending,
• aṣ-ṣabbāra

So the ص is doubled in pronunciation.

In this sentence, the order is:

• صديقي = subject
• يسقي = verb
• الصبار = object

So the structure is subject–verb–object:

• My friend — waters — the cactus

Also, the meaning strongly supports this reading: a friend is the one doing the watering, and the cactus is receiving the action.

Yes.

Arabic commonly allows both:

• صديقي يسقي الصبار
• يسقي صديقي الصبار

Both can mean My friend waters the cactus.

A rough difference is:

• صديقي يسقي الصبار: starts with my friend, so it can feel a bit more subject-focused
• يسقي صديقي الصبار: starts with the verb, which is also very normal in Arabic, especially in more formal styles

In fully inflected MSA, you could write the sentence as:

• صَدِيقِي يَسْقِي الصَّبَّارَ

Here:

• الصبّارَ is the direct object, so it takes fatḥa in full reading
• صديقي is the subject, so it is nominative, but that ending is not clearly visible because of the attached ـي

In normal pauses at the end of a sentence, many speakers do not pronounce the final short vowel, so you will often just hear aṣ-ṣabbār.

Yes.

• صديق comes from the root ص د ق, which is connected with truthfulness and sincerity
• سقى / يسقي comes from س ق ي, related to giving water or drink
• الصبّار is traditionally connected with ص ب ر, the root associated with patience/endurance

For learners, roots are very helpful because they let you notice families of related words across Arabic.