يستمر المطر اليوم.

Because Arabic often uses verb–subject order, especially in simple statements.

So يستمر المطر اليوم is a very normal Arabic pattern:

• يستمر = the verb
• المطر = the subject
• اليوم = a time expression

A more English-like order, المطر يستمر اليوم, is also possible, but it changes the feel slightly by putting more focus on the rain.

يستمر is in the present/imperfect form.

In Modern Standard Arabic, this form can express:

• present: it continues
• habitual action: it usually continues
• sometimes near future, depending on context

In this sentence, with اليوم, it is naturally understood as a present-time statement.

Arabic often does not use a present-tense verb equivalent to is/are in simple nominal sentences.

However, here the sentence already has a main verb: يستمر = continues.

So English might say:

• The rain continues today and Arabic simply says:
• يستمر المطر اليوم

No extra word for is is needed.

المطر means the rain, with الـ meaning the.

In Arabic, weather nouns are often used as definite when speaking about a known or general phenomenon, so المطر sounds natural here.

If you said مطر without الـ, it would usually sound less natural in this kind of sentence unless the context specifically called for an indefinite noun.

اليوم means today and functions as a time expression.

It tells you when the action happens:

• يستمر = continues
• اليوم = today

Arabic often places time expressions at the end of the sentence, as in this example, though other positions are also possible for emphasis.

A full vowel-marked version is:

يَسْتَمِرُّ الْمَطَرُ الْيَوْمَ

This shows:

• يَسْتَمِرُّ = present verb, ending in ـُ
• الْمَطَرُ = subject in the nominative case
• الْيَوْمَ = adverbial time expression, often in the accusative

In everyday printed Arabic, these short vowels are usually omitted.

Arabic is expressing the idea through the noun rain itself, not through an impersonal it.

So instead of saying something like it rains, Arabic says:

• the rain continues
• the rain falls
• the rain increases

That is why المطر appears as the real grammatical subject.

Yes. المطر is grammatically masculine singular, so the verb matches it.

Because the verb comes before the subject here, Arabic normally uses a singular verb form, which is exactly what we see in يستمر.

Yes, you can.

Both are grammatically correct:

• يستمر المطر اليوم
• المطر يستمر اليوم

The first is a standard verbal sentence and is often the more neutral way to report an event.

The second gives a little more prominence to المطر, because it starts with the subject.

يستمر comes from the verb استمرّ, which means to continue.

It is commonly analyzed as coming from the root م ر ر.

So:

• past: استمرّ
• present: يستمرّ

For a learner, the most useful thing is to recognize يستمر as he/it continues in the present tense. In this sentence, it refers to المطر.