כשחם מאוד במשרד, אני מפעילה את המזגן.

כש means when here.

In כשחם מאוד במשרד, it introduces a time clause:

• כשחם מאוד = when it’s very hot

A useful thing to notice is that כש is attached directly to the next word in writing:

• כש + חם = כשחם

This is very common in Hebrew.

In Hebrew, the verb to be is usually not expressed in the present tense.

So instead of saying something literally like when it is very hot in the office, Hebrew says:

• כשחם מאוד במשרד
• literally: when very hot in the office

This is normal Hebrew grammar. The idea of is is understood automatically.

In weather and temperature expressions, Hebrew often uses חם in an impersonal way, similar to English it’s hot.

So:

• חם = hot
• קר = cold

Even though many Hebrew adjectives change for gender, in this kind of expression you normally use:

• חם = it’s hot
• not חמה

So כשחם מאוד simply means when it’s very hot.

מאוד means very.

In Hebrew, מאוד usually comes after the adjective or adverb it modifies:

• חם מאוד = very hot
• literally: hot very

That word order is normal in Hebrew.

Because ב־ is a prefix meaning in / at, and Hebrew often attaches prepositions directly to the following noun.

So:

• ב + משרד = במשרד
• in the office / in an office

This is standard Hebrew spelling.

There is also a useful extra point here: in unpointed Hebrew, במשרד can represent either:

• בְמשרד = in an office
• בַמשרד = in the office

They are written the same without vowel marks, so context tells you which one is meant. In your sentence, the meaning given is in the office.

Because מפעילה is the feminine singular form, and מפעיל is the masculine singular form.

So:

• אני מפעילה = I turn on / I activate said by a woman
• אני מפעיל = I turn on / I activate said by a man

Hebrew present-tense verbs agree with the gender and number of the subject.

מפעילה comes from the verb להפעיל, which means to activate, to operate, or to turn on.

In this sentence, the most natural English translation is:

• I turn on the air conditioner

With machines and devices, להפעיל is very common. It can sound a little more like activate/operate than switch on, but in everyday English turn on is usually the best translation here.

את is the direct object marker.

Hebrew uses את before a direct object that is definite, such as:

• a noun with ה־ (the)
• a proper name
• a pronoun in some structures

So:

• אני מפעילה את המזגן
• literally: I turn on [direct object marker] the air conditioner

It does not mean you here. That is a different word, even though it is written the same way in unpointed Hebrew.

Because המזגן means the air conditioner, which is a definite object.

In Hebrew, when the direct object is definite, you normally use את:

• אני מפעילה את המזגן = I turn on the air conditioner

If it were indefinite, you would usually leave out את:

• אני מפעילה מזגן = I turn on an air conditioner
  Though in real life, that version is less natural in this context unless you specifically mean an air conditioner rather than a known one.

מזגן means air conditioner.

With the definite article:

• המזגן = the air conditioner

So the phrase:

• את המזגן means:
• the air conditioner as the direct object of the verb

It is most naturally understood as a habitual or general action:

• When it’s very hot in the office, I turn on the air conditioner.

Even though מפעילה is a present-tense form, Hebrew present tense often also covers:

• habitual actions
• general truths
• repeated behavior

So here it means what the speaker usually does in that situation, not necessarily what she is doing at this exact second.

Yes. You can also say:

• אני מפעילה את המזגן כשחם מאוד במשרד.

That means the same thing:

• I turn on the air conditioner when it’s very hot in the office.

The version with כשחם מאוד במשרד first puts a little more focus on the condition or situation. Both are natural.